Retarder and optical head device installing the same

ABSTRACT

An adhesive is coated on at least one surface of a thin film of organic material having birefringent properties and a phase-difference producing function, and a fixing substrate having transmitting or reflecting properties is bonded to the thin film of organic material by the adhesive, wherein materials satisfying relations of E 1 &lt;E 2  and E 3 &lt;E 2  where E 1  is the linear expansion coefficient of the thin film of organic material, E 2  is the linear expansion coefficient of the adhesive and E 3  is the linear expansion coefficient of the fixing substrate, and the glass transition temperature of the thin film of organic material being 150° or more, are selected. 
     With such structure, temperature characteristics of a phase difference can be obtained so as to reduce a temperature change of an oscillated wavelength emitted from a semiconductor laser source whereby the phase difference can be kept constant even when there is a fluctuation in a wavelength of light emitted from the semiconductor laser source due to temperature.

TECHNICAL FIELD

The present invention relates to a retarder used for an optical headdevice for recording or reproducing information by irradiating light toan optical recording medium and an optical head device installing theretarder.

BACKGROUND ART

An optical head device provided with an optical element such as aretarder, a diffraction element and so on has been used in order towrite (Hereinbelow, referred to as record) information in an opticalrecording medium such as an optical disk, e.g., CD, DVD or the like, ora magneto-optical disk (Hereinbelow, referred to as an optical disk), orread out (Hereinbelow, referred to as reproduce) information from theoptical recording medium.

The optical head device is to introduce laser light emitted from asemiconductor laser light source to the optical disk by converging thelaser light by means of an objective lens, and to detect informationrecorded in the optical disk by receiving reflected light from theoptical disk by a photodetector.

In an optical path from the semiconductor laser source to the opticaldisk or an optical path from the optical disk to the photodetector, anoptical element such as a diffraction element, a beam splitter or thelike and a retarder for changing a state of polarization of laser lightare properly arranged to obtain highly accurate, stablerecording/reproducing. In particular, by using a combination of theoptical element in which the characteristics are changed depending on astate of polarization of laser light and the retarder for changing astate of polarization, an optical system having a high utilizationefficiency of light can be realized, and flexibility in designing theoptical system can be improved. Accordingly, various types of opticalsystem for optical head devices have been proposed.

For the retarder, such one prepared by polishing a single crystal ofinorganic substance such as quartz has conventionally been used.However, since the single crystal of inorganic substance has largeincident angle dependence on a phase difference of transmitted light,the single crystal is improper as the retarder for generating a desiredphase difference. Further, in an optical element made of the singlecrystal of inorganic substance, the number of steps in manufacture islarge. Accordingly, such one having a thin film of organic material madeof, for example, uniaxially stretched polycarbonate, having birefringentproperties has been proposed. In cases of using the conventionalretarder formed by polishing the single crystal of inorganic substancesuch as quartz and the retarder comprising an organic material, therewere the problems as described below, in common.

As a first problem, there is the temperature dependence of the wavefrontaberration of transmitted light from the retarder. When an optical headdevice is operated for a long time, the temperature in the optical headdevice rises with a lapse of time. Further, a semiconductor laser sourceused for the optical head device has generally such temperaturecharacteristics that the oscillated wavelength increases with atemperature rise. In the optical head device installing therein theretarder, an oscillated wavelength from a semiconductor laser sourcefluctuates due to a temperature change, a predetermined phase differenceat the time when the laser light passes through the retarder can not beobtained, and an adverse influence such as a reduction of signalintensity or an increase of noises in signals may be caused inrecording/reproducing an optical disk.

In the quartz or the thin film of organic material, the fluctuation ofthe optical characteristics due to a temperature change is generallysmall. Accordingly, when the oscillated wavelength from thesemiconductor laser source fluctuates due to a temperature change, afluctuation in the wavelength of the laser light could not becompensated by the retarder in which the above-mentioned material wasused.

Further, since the linear expansion coefficient of a fixing substrateused in the retarder was generally different from the linear expansioncoefficient of a thin film of organic material, the thin film of organicmaterial deformed with a temperature rise, disturbances in theretardation value of the retarder and the retardation axis (fast axis)direction took place, and further, the smoothness of the thin film oforganic material was reduced, whereby there was a problem of causing thedisturbance of the wavefront of the laser light passing through theretarder.

As a second problem, it was difficult to minimize the size of the devicebecause of an increased number of parts in assembling the retarder inthe optical head device. In the retarder formed by polishing the singlecrystal of inorganic substance such as quartz, the retarder had to belocated in a region of parallel light in an optical path in the opticalhead device because the incident angle dependence of phase differencewas large, whereby there was a problem that the surface area of theretarder became large. Further, in the retarder made of an organicmaterial, although the incident angle dependence of phase difference wassmall, the thickness of the retarder formed by bonding on the substratewas increased as a whole. Accordingly, it was difficult to minimize thesize and the weight of the optical head device.

As a third problem, there is the wavelength dependence in a state ofpolarization of transmitted light in the retarder. Since the recordingdensity can be increased by shortening the wavelength of usable laserlight in an optical head device, an attempt of shortening the wavelengthhas been made on light sources. On the other hand, it is necessary toperform reproduction by using widespread laser light having a wavelengthband of 790 nm for many compact optical disks (Hereinbelow, referred toas CD), and various optical head systems which allow the compatibilitybetween laser light having a shorter wavelength and the ordinary laserlight having a wavelength band of 790 nm have been proposed. In order toassure the compatibility with respect to the ordinary optical disks,there is a system in which the ordinary laser light source having awavelength band of 790 nm is disposed in addition to a short wavelengthlaser light source having a wavelength band of 650 nm for digitalversatile disks for high-density recording (Hereinbelow, referred to asDVD).

Further, as a semiconductor laser source for emitting light havingtwo-wavelengths, a two-wavelength semiconductor laser source ofmonolithic structure in which a semiconductor laser source having awavelength band of 790 nm and a semiconductor laser source having awavelength band of 650 nm are formed in one chip, or a two-wavelengthsemiconductor laser source comprising a plurality of chips in whichlaser chips having different wavelength bands are disposed with aninterval of light emitting point of about 100-300 μm, for example. Withsuch two-wavelength semiconductor laser sources, the number of parts canbe reduced to minimize the size in comparison with conventional opticalhead devices having two-semiconductor laser sources as separate units.Further, in the case of the two-wavelength semiconductor laser source ofmonolithic structure, accuracy in positioning light emitting points canbe improved, and assembling and adjusting are simplified, whereby thecharacteristics of optical head capable of providing stablerecording/reproducing information are easily obtainable.

In a case that a retarder is used for a conventional optical head deviceusing only a single wavelength, it is easy to prepare a quarter-waveplate providing a phase difference of π/2 or a half-wave plate forproviding a phase difference of π with respect to linearly polarizedincident light. However, when a retarder is used for an optical headdevice using two-wavelengths, it was difficult for the conventionalretarder to provide optical characteristics of such technical level thatthe quarter-wave plate was usable for a wavelength band of 650 nm and awavelength band of 790 nm, or the half-wave plate was usable for awavelength band of 650 nm and a wavelength band of 790 nm.

Accordingly, for an optical head device in which laser light ofdifferent wavelengths are used for DVD and CD, it was necessary to usedifferent retarders for different wavelengths separately whereby thesize of the device was increased. Further, in a case of using thetwo-wavelength semiconductor laser source, it was necessary to produce adesired phase difference with respect to two different kinds ofwavelength by a single retarder, and therefore, a retarder capable ofproducing a desired phase difference depending on different wavelengthswas expected.

It is an object of the present invention to provide a retarder capableof compensating a fluctuation of wavelength caused by a temperaturechange of light emitted from a semiconductor laser source, inparticular, capable of producing stably a predetermined phase differenceeven in a high temperature region, and an optical element provided withsuch retarder.

Further, it is another object of the present invention to provide anoptical element wherein a retarder to be installed in an optical headdevice has a plurality of functions, whereby the optical head device canbe miniaturized.

Further, it is another object of the present invention to provide aretarder for producing a desired phase difference depending on differentwavelengths and an optical element provided with such retarder in anoptical head device wherein laser light having two or more differentwavelengths are used as light sources.

DISCLOSURE OF THE INVENTION

In accordance with the present invention, there is provided a retardercomprising a thin film of organic material having birefringentproperties, an adhesive coated on at least one surface of the thin filmof organic material, and a fixing substrate having transmitting orreflecting properties, bonded to the thin film of organic material bythe adhesive, wherein among the linear expansion coefficient E₁ of thethin film of organic material, the linear expansion coefficient E₂ ofthe adhesive and the linear expansion coefficient E₃ of the fixingsubstrate, relations of E₁<E₂ and E₃<E₂ are satisfied.

Further, the present invention is to provide the above-mentionedretarder, wherein the thin film of organic material includes at leastone selected from the group consisting of polycarbonate, polyimide,polyallylate, polyethersulfone, (alicyclic) polyolefin,poly(meth)acrylate, polyetherimide and polymerized liquid crystal.

Further, the present invention is to provide the above-mentionedretarder, wherein the adhesive includes at least one selected from thegroup consisting of acryl type, epoxy type, urethane type and polyestertype.

Further, there is provided an optical head device comprising asemiconductor laser light source, an objective lens for converging laserlight emitted from the semiconductor laser light source, an opticalrecording medium to which the laser light is converged and introduced, aphotodetector for receiving reflected light from the optical recordingmedium and, an optical element fabricated by combining theabove-mentioned retarder with a polarizing diffraction element havingdifferent diffraction efficiencies depending on a state of polarizationof incident light wherein the optical element is located in an opticalpath from the laser light source to the optical recording medium, or anoptical path from the recording medium to the photodetector.

Further, there is provided an optical element wherein a structure havingat least one element among the following three elements is formed on afixing substrate on which the above-mentioned retarder is formed:

(1) an aperture controlling element provided with a first region in acentral portion, which transmits light having two or more kinds ofwavelength and a second region surrounding the first region, whichreflects or diffracts light having one or more kinds of wavelength,

(2) a retarder having a ringed belt-like groove for correcting thewavefront of transmitted light in a central portion, which transmitslight having two or more kinds of wavelength, and

(3) a diffraction element having a periodic concave and convex portionin cross-sectional view, which diffracts incident light.

Further, the present invention is to provide an optical head devicewherein laser light emitted from a semiconductor laser source isconverged by an objective lens to be introduced into an opticalrecording medium, and reflected light from the optical recording mediumis received by a photodetector, the optical head device beingcharacterized in that the above-mentioned optical element is located inan optical path from the laser light source to the optical recordingmedium, or a light path from the optical recording medium to thephotodetector.

Further, the present invention is to provide an optical head devicecomprising a semiconductor laser light source for emitting laser lighthaving two or more different wavelengths, an objective lens forconverging the laser light emitted from the semiconductor laser lightsource, an optical recording medium to which the laser light isconverged and introduced, a photodetector for receiving reflected lightfrom the optical recording medium, and a broadband retarder located inan optical path from the laser light source to the optical recordingmedium, or an optical path from the optical recording medium to thephotodetector to control a state of phase of the laser light, thebroadband retarder having such structure that a phase plate A to whicheither one laser light is first incident and a phase plate B to whichsaid laser light is secondly incident are piled up so that therespective optical axes are crossed wherein the ratio of the retardationvalue of the phase plate A to the retardation value of the phase plate Bis 1.8-2.2.

Further, the present invention is to provide the optical head devicedescribed just above, wherein the laser light is two kinds of laserlight having different wavelengths, the ratio of the retardation valuesis 2, and the degrees of elliptical polarization when the two kinds oflaser light are transmitted through the broadband retarder aresubstantially equal.

Further, the present invention is to provide an optical head devicecomprising a semiconductor laser light source for emitting linearlypolarized light having wavelengths of λ₁ and λ₂ (λ₁<λ₂), an objectivelens for converging the laser light emitted from the semiconductor lasersource, an optical recording medium to which the laser light isconverged and introduced, a photodetector for receiving reflected lightfrom the optical recording medium, and the above-mentioned retarderlocated in an optical path from the laser light source to the opticalrecording medium wherein the linearly polarized light having wavelengthsof λ₁ and λ₂ are incident to the retarder, two thin films of organicmaterial having birefringent properties each having a retardation valueof λ/2 with respect to linearly polarized light having a wavelength of λin a relation of λ₁≦λ≦λ₂ are piled up so that the respective opticalaxes are crossed, and when the linearly polarized light havingwavelengths of λ₁ and λ₂ are transmitted through the thin films oforganic material, the planes of polarized light provided by the linearlypolarized light are rotated by the same angle.

Further, the present invention is to provide an optical head devicecomprising a semiconductor laser light source for emitting two kinds oflinearly polarized light having different wavelengths and the planes ofpolarized light in parallel to each other, an objective lens forconverging the laser light emitted from the semiconductor laser source,an optical recording medium to which the laser light is converged andintroduced, a photodetector for receiving reflected light from theoptical recording medium, and the above-mentioned retarder located in anoptical path from the laser light source to the optical recordingmedium, or an optical path from the optical recording medium to thephotodetector, wherein the retarder comprises a thin film of organicmaterial to produce a phase difference of 2π (m₁−½) (m₁ is a naturalnumber) with respect to a kind of linearly polarized light and a phasedifference of 2π m₂ (m₂ is a natural number) with respect to the otherkind of linearly polarized light when the two kinds of linearlypolarized light having different wavelengths are transmittedtherethrough with an inclination of 45° in a polarization direction inits fast axis direction, whereby the planes of polarized light providedby the linearly polarized light of two kinds of wavelengths crossperpendicularly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing the structure of the retarderaccording to a first embodiment of the present invention.

FIG. 2 is a graph showing the fluctuation of retardation valuecorresponding to the fluctuation of the wavelength of oscillated lightfrom a semiconductor laser source and the fluctuation of retardationvalues in the retarder vs. temperature change.

FIG. 3 is a cross-sectional view showing the structure of the retarderaccording to a second embodiment of the present invention.

FIG. 4 is a cross-sectional view showing the structure of the polarizingdiffraction type retarder according to a third embodiment wherein theretarder of the present invention and a polarizing diffraction elementare unified.

FIG. 5 is a cross-sectional view showing an example of the structure ofthe reflection type retarder according to a fourth embodiment of thepresent invention.

FIG. 6 is a cross-sectional view showing another example of thestructure of the reflection type retarder according to the fourthembodiment of the present invention.

FIG. 7 is a cross-sectional view showing another example of thestructure of the reflection type retarder according to the fourthembodiment of the present invention.

FIG. 8 is a cross-sectional view showing the structure of the opticalelement according to a fifth embodiment of the present invention.

FIG. 9 are diagrams showing the structure of the optical elementaccording to a sixth embodiment of the present invention wherein (a) isa cross-sectional view showing an example of the structure of theoptical element, (b) is a cross-sectional view showing another exampleof the structure of the optical element, and (c) is a plan view of theoptical element of (a) and (b) observed from an upper side of thedrawing.

FIG. 10 are diagrams showing the structure of the broadband phase plateaccording to a seventh embodiment of the present invention wherein (a)is a cross-sectional view in a state that two phase plates are piled up,and (b) is a plan view showing an angular relation of each optical axisof the two piled phase plates.

FIG. 11 is a graph showing changes of the ellipse ratio angle oftransmitted light vs. set angles (θ₁, θ₂) of the broadband phase plateof the seventh embodiment of the present invention wherein (a) is agraph in a case that θ₂ is larger than θ₁, and (b) is a graph in a casethat θ₁ is larger than θ₂.

FIG. 12 is a graph showing the wavelength dependence vs. the ellipseratio angle of transmitted light in the broadband phase plate of theseventh embodiment of the present invention.

FIG. 13 illustrates graphs showing set angles (θ₁, θ₂) of the broadbandphase plate of the seventh embodiment of the present invention in a casethat the phase plate functions as a quarter-wave plate with respect tolight having two different wavelengths wherein (a) is a graph in a casethat θ₁=15+a and θ₂=75−a where a is 10° or less, and (b) is a case thatθ₁=75−a and θ₂=15+a where a is 10° or less.

FIG. 14 is a graph showing the wavelength dependence of the ellipseratio angle of transmitted light in the broadband phase plate of theseventh embodiment of the present invention (in a case of a=0 and a=3.2in FIG. 13(b)).

FIG. 15 is a diagram showing an example of the structure of an opticalhead device in which the broadband phase plate of the seventh embodimentof the present invention is installed.

FIG. 16 illustrates cross-sectional views showing the structure of theretarder according to an eighth embodiment of the present inventionwherein (a) is a diagram showing a state of the rotation of thepolarization direction in the incidence of linearly polarized lighthaving a wavelength and (b) is a diagram showing a state of the rotationof the polarization direction in the incidence of linearly polarizedlight having another wavelength.

FIG. 17 is a diagram showing the structure of an optical head device inwhich the retarder of the eighth embodiment of the present invention isinstalled.

FIG. 18 is a cross-sectional view showing the structure of the retarderaccording to a ninth embodiment of the present invention.

FIG. 19 illustrates cross-sectional views showing the structure of theretarder according to a tenth embodiment of the present inventionwherein (a) is a cross-sectional view showing a state that linearlypolarized light having a wavelength is incident to the transparentsubstrate with its optical axis perpendicular to the substrate and (b)is a cross-sectional view showing a state that linearly polarized lighthaving another wavelength is incident to the transparent substrate withits optical axis oblique to the substrate.

FIG. 20 is a diagram showing the optical head device according to aneleventh embodiment of the present invention.

FIG. 21 is a side view showing diagrammatically the structure of anoptical head device in which the polarizing diffraction type retardershown in FIG. 4 is installed.

FIG. 22 is a diagram showing a measuring optical system and a coordinatesystem of the reflection type retarder of the present invention.

FIG. 23 illustrates diagrams showing the structure of a polarizingdiffraction element with the broadband phase plate of the seventhembodiment of the present invention wherein (a) is a cross-sectionalview showing piled broadband phase plate and the polarizing diffractionelement and (b) is a plan view showing an angular relation of eachoptical axis and so on of two piled phase plates.

FIG. 24 is a graph showing the wavelength dependence of the ellipseratio angle of light converged to an optical disk in the retarder of theeighth embodiment of the present invention as well as that in aconventional example.

FIG. 25 is a graph showing the wavelength dependence of the polarizationdirection of light converged to an optical disk in the retarder of theeighth embodiment of the present invention as well as that in aconventional example.

FIG. 26 illustrates diagrams showing the structure of the two-wavelengthdiffraction element of the sixth embodiment of the present inventionwherein (a) is a diagram showing a light path of incident light having awavelength and (b) is a diagram showing an optical path of incidentlight having another wavelength.

FIG. 27 illustrates diagrams showing the structure of the two-wavelengthdiffraction element of the ninth embodiment of the present inventionwherein (a) is a diagram showing an optical path of incident lighthaving a wavelength and (b) is a diagram showing an optical path ofincident light having another wavelength.

BEST MODE FOR CARRYING OUT THE INVENTION

[Embodiment 1]

In the retarder of the present invention, an adhesive is coated on atlease one surface of a thin film of organic material having birefringentproperties and the thin film of organic material is bonded by theadhesive to a fixing substrate having transmitting or reflectingproperties.

FIG. 1 is a cross-sectional view showing the structure of a retarder 1Aaccording to a first embodiment. In the first embodiment, a thin filmhaving birefringent properties, i.e., a phase-difference producingfunction is fixed between two opposed fixing substrates by an adhesive.As the thin film of organic material used for the retarder, a thin filmof organic material formed by uniaxially stretching a thin film ofpolymer such as polycarbonate, polyimide, polyallylate,polyethersulfone, (alicyclic) polyolefin, poly(meth)acrylate or the liketo provide birefringent properties, hence, a phase-difference producingfunction, may be used.

Although resins other than these resins may also be used as such a thinfilm having a phase-difference producing function, it is preferred, in aview of heat resistance, to use (modified) polycarbonate, polyimide,polyallylate, polyethersulfone, (alicyclic) polyolefin,poly(meth)acrylate or polyetherimide. For industrial use as massproduction product, polycarbonate is more preferable as the stability isgood. A thin film of polymerized liquid crystal obtained by coating amonomer of polymerized liquid crystal onto a fixing substrate, theliquid crystal being subjected to a conventional aligning treatment,followed by curing can be used as a thin film of organic material. Theabove-mentioned polymerized liquid crystal may be either of a side chaintype or a main chain type. When the glass transition temperature of athin film of organic material is 150° C. or more, deterioration in thebirefringent properties, i.e., the phase-difference producing propertiesof the thin film of organic material, due to a temperature rise canpreferably be prevented.

As material for the adhesive, acryl type, epoxy type, urethane type,polyester type, polyimide type, urea type, melamine type, furan resintype, isocyanate type, silicone type, cellulose type, vinyl acetatetype, vinyl chloride type, rubber type or a mixture thereof may be used.An adhesive of a UV curing type or a thermosettable type is preferablebecause workability is good. However, the adhesive is not limited to theabove. Further, the adhesive should be coated smoothly as thin aspossible with a constant thickness in order to minimize the wavefrontaberration. For example, it is preferable to finish smoothly so that thetransmission wavefront aberration of the retarder is 0.02 λ_(rms) orless (λ is a wavelength and rms is a root-mean-square value). As amethod of coating, a method for spin coating or roller coating ispreferable because workability is good and control to the thickness iseasy.

As the fixing substrate having a transmitting function, a smooth opticalglass is preferred. Further, as the fixing substrate having a reflectingfunction, it is possible to use an optical glass on which a reflectingsurface is formed by using a thin film of aluminum or another metal onits surface. Further, in order to reduce a difference between therefractive index of the adhesive and the refractive index of the thinfilm of organic material having a phase-difference producing function; adisturbance of the wavefront of transmitted light generated depending onirregularity of the surface of the thin film of organic material, and aloss of reflection at the interface between the adhesive and the thinfilm of organic material, it is preferable to use an adhesive having arefractive index which is substantially equal to the refractive index ofthe thin film of organic material.

As shown in FIG. 1, surfaces of a thin film of organic material 101 of,e.g., uniaxially stretched polycarbonate are fixed to fixing substrates104, 105 such as glass having smoothened surfaces and having atransmitting function, by adhesive layers 102, 103 of, e.g., an acrylictype. In the structure of FIG. 1, both surfaces of the thin film oforganic material 101 are sandwiched and fixed by the two fixingsubstrates. The thin film of organic material 101 is so formed as toprovide retardation values, e.g., λ/4, 5λ/4, λ/2 and so on with respectto transmitted laser light having a wavelength of λ emitted from asemiconductor laser light source.

In the thin film of organic material such as polycarbonate, the polymerchain is oriented by being stretched uniaxially so that there is adifference between the refractive index in a stretched direction and therefractive index in a direction perpendicular to the stretcheddirection, whereby the thin film of organic material having aphase-difference producing function given by birefringent properties isprovided. It has been said that the thermal expansion coefficient of thethin film of polycarbonate is generally isotropic in the film plane, andthe temperature characteristic of the retardation values (a fluctuationquantity in the retardation values relying on temperature) is small. Inthe experiments conducted by the applicant, it has been found that thetemperature characteristic of the retardation values can be improved bybonding and fixing the thin film of polycarbonate on a transparentsubstrate such as glass.

As shown is FIG. 2, in the retarder of the present invention in whichthe retarder is fixed to transparent substrates by an adhesive (▪ inFIG. 2), the wavelength functioning as a retarder becomes large with atemperature rise because the retardation value increases with atemperature rise, in comparison with a case of the thin film ofpolycarbonate used solely (▴ in FIG. 2). Since the wavelength of lightoscillated from a semiconductor laser source generally increases with atemperature rise, it is preferable that the retardation value increaseswith a temperature rise (● in FIG. 2) in order to make the phasedifference (2π×retardation value/wavelength) constant to a fluctuationof the wavelength of laser light.

In the retarder of the present invention, the temperature characteristicof retardation values is resulted in the same direction as a fluctuationof a wavelength oscillated from a semiconductor laser source, andfluctuation quantities of a wavelength of oscillated laser light and afluctuation of retardation values of the retarder well correspond to thetemperature characteristic. Therefore, a change of phase difference dueto a fluctuation of the wavelength of laser light can be suppressed by afluctuation of temperature of retardation values of the retarder.Accordingly, with use of the retarder of the present invention, it ispossible to compensate the fluctuation of a phase difference caused bythe fluctuation of a wavelength due to the temperature of light emittedfrom the semiconductor laser source. Namely, the above-mentioned problemthat a predetermined phase difference can not be obtained because of thetemperature characteristic of the semiconductor laser source, can besolved.

In further study by the applicant, it has been found that expansions ofthe thin film of organic material and the fixing substrate with atemperature rise can be absorbed by the adhesive, and the deformation ofthe thin film of organic material can be prevented when the linearexpansion coefficient E₁ (/° C.) of the thin film of organic material,the linear expansion coefficient E₂ (/° C.) of the adhesive, and thelinear expansion coefficient E₃ (/° C.) of the fixing substrate as thecharacteristics of thermal expansion coefficient of each material whichconstitutes the retarder satisfy the relation of Formula (1).E ₁ <E ₂ and E ₃ <E ₂  (1)

As a result, the retarder in which there is little possibility ofcausing a disturbance of the wavefront of laser light passing throughthe retarder due to a change of the retardation value, a change of theretardation axis direction and a deformation of the thin film of organicmaterial, can be provided,

In this embodiment, polycarbonate (linear expansion coefficientE₁=6.2×10⁻⁶/° C.) was used as the thin film of organic material, apolyester type adhesive (linear expansion coefficient E₂=1.2×10⁻⁴/° C.)was used as the adhesive and a glass substrate (linear expansioncoefficient E₃=95×10⁻⁷/° C.) was used as the fixing substrate. Since theretarder having such structure satisfies the relation of theabove-mentioned Formula (1), and can absorb expansions of the thin filmof organic material and the fixing substrate by the adhesive in responseto a temperature change, the expansions of the thin film of organicmaterial and the fixing substrate caused by a temperature rise, inparticular, even in a high temperature region, can be absorbed by theadhesive, and stable, excellent phase-difference properties can beexhibited without causing a disturbance of the wavefront of laser lightpassing through the thin film of organic material and the fixingsubstrate which is caused by a change of the phase difference, a changeof the retardation axis direction and a deformation of the thin film oforganic material, or without causing an irreversible change such aspeeling.

In several embodiments described below, explanation will be made as tovarious retarders of the present invention having structural elementsfor the retarders, and optical head devices using these retarders.

[Embodiment 2]

FIG. 3 is a cross-sectional view showing the structure of a retarder 1Baccording to a second embodiment. In the second embodiment, A thin filmof organic material 101 having birefringent properties such as a thinfilm of polycarbonate, having one surface on which an adhesive 102 of,e.g., a polyester type is coated, is fixed to a fixing substrate 104made of glass having a smooth surface. Even though the retarder is ofsuch structure that one surface of the thin film of organic material 101is fixed to the single fixing substrate 104 by the adhesive 102, thereis obtainable such effect of compensating a fluctuation of phasedifference of the retarder caused by a fluctuation of temperature of anoscillated wavelength from a semiconductor laser element in the samemanner as in the first embodiment. However, the first embodiment havingthe structure that both surfaces of the retarder are fixed can providemore remarkable effect.

Further, in a case that the retarder of the present invention is used asan optical element for, e.g., an optical head device, it is preferableto unify by piling the retarder on a polarizing diffraction elementshowing different diffraction efficiencies depending on polarizationdirections of incident light. With this, the optical element functioningas a polarizing diffraction element and a retarder is provided, and aspace to be occupied in the optical head device can be reduced tothereby allow miniaturization.

[Embodiment 3]

FIG. 4 is a cross-sectional view showing the structure of a polarizingdiffraction type retarder according to a third embodiment wherein theretarder 1B of the present invention and a polarizing diffractionelement 2 are unified. In the third embodiment, on the polarizingdiffraction element 2 comprising a polarizing diffraction grating 107formed on a fixing substrate 105 having transmitting properties, onesurface of a thin film of organic material 101 having birefringentproperties is fixed by an adhesive 103. A fixing substrate 104 havingtransmitting properties is fixed on the other surface of the thin filmof organic material 101 by an adhesive 102. Thus, the polarizingdiffraction element 2 and the retarder 1B including the thin film oforganic material 101 are unified by the adhesive 103. As the polarizingdiffraction element used here, there are a diffraction element using anorganic material having birefringent properties such as liquid crystal,a polymerized liquid crystal or the like, or a diffraction element usinga single crystal of inorganic material having birefringent propertiessuch as LiNbO₃ or the like.

When the polarizing diffraction type retarder shown in FIG. 4 is usedfor an optical head device for recording or reproducing information onan optical disk such as CD or DVD, it is located in an optical pathbetween the optical disk and a photodetector for detecting reproducingsignals so that the retarder 1B is at a side of the optical disk. Inthis case, the retardation value of the thin film of organic material101 is determined to be λ/4 or 5λ/4 with respect to, for example, awavelength band λ of 650 nm from a semiconductor laser light source.With this, linearly polarized laser light emitted from the semiconductorlaser source rotates 90° in polarization direction in going andreturning paths while the laser light goes and returns through the thinfilm of organic material.

In this case, the polarizing diffraction element used is such one havinga high transmittance with respect to light having a polarizationdirection in a going path and having a high diffraction efficiency withrespect to light having a polarization direction in a returning path,whereby an optical head device having a high utilization efficiency oflight in going and returning the paths between the semiconductor lasersource and the photodetector, can be constituted.

[Embodiment 4]

FIG. 5 shows a retarder 20A, in cross-section, according to a fourthembodiment of the present invention. A reflection type retardercomprises a thin film of organic material 201 having birefringentproperties, which has a surface fixed to a fixing substrate 204 having areflecting function by an adhesive 202. Further, on the other surface ofthe thin film of organic material 201, an anti-reflection film 206 isprovided. In the reflection type retarder, since incident light isreflected by the fixing substrate to pass twice through the thin film oforganic material, the film thickness of the thin film of organicmaterial can be reduced to about half in comparison with a transmittingtype retarder. Further, the thin film of organic material 201 can be soformed that when laser light from a semiconductor laser source or thelike is incident perpendicularly and transmits therethrough, a phasedifference of mπ/4 (m is a natural number) is produced with respect tothe transmitting light, and when the light is reflected by the fixingsubstrate 204, a phase difference of mπ/2 is produced with respect tothe reflected light.

As the fixing substrate 204 having a reflecting function used for thereflection type retarder 20A according to this embodiment, such onecomprising an optical glass having a smooth surface on which a thin filmof metal such as aluminum or a multi-layer thin film made of dielectricmaterials such as TiO₂/SiO₂ or Ta₂O₅/SiO₂, may be used. Further, in thefilm structure comprising the multi-layer thin film made of dielectricmaterials, a dichroic mirror which reflects light having a specifiedwavelength band region to bend the optical axis 90°, and to allow lighthaving another wavelength band region to pass through, may be used.

FIG. 6 shows a retarder 20B according to another constitution example ofthis embodiment. It has such structure that instead of thenon-reflective coat 206 provided on one surface of the thin film oforganic material shown in FIG. 5, an optical cover glass as a smoothfixing substrate 205 provided with an anti-reflection film (not shown)is bonded by an adhesive 203, so that the wavefront aberration resultedfrom the surface roughness of the thin film of organic material 201 canbe reduced. Reference numeral 203 designates an adhesive and referencenumeral 204 designates a fixing substrate.

FIG. 7 shows a retarder 20C according to another constitution example ofthis embodiment. In this embodiment, each shape of an optical coverglass 208 and a fixing substrate 207 having a reflecting function is ina prism-like shape. When a flat plate-like substrate having parallelsurfaces is used, astigmatism is produced in proportion to the thicknessof the substrate with respect to divergent or convergent incident light.However, by providing such prism-like structure, occurrence of theastigmatism can be suppressed. The other same reference numerals as inFIG. 6 designate the same elements as in FIG. 6.

When the retarders according to the fourth embodiment are used for anoptical head device for recording or reproducing information in anoptical disk such as CD, DVD or the like, arrangement should be made sothat light emitted from a semiconductor laser source is incidentobliquely to the thin film of organic material 201. By using a thin filmof organic material producing a phase difference having a value in thevicinity of (2p−1)π/2 (p is a natural number) in a case that laser lightis incident perpendicularly to the thin film of organic material andemits after being reflected by the fixing substrate, linearly polarizedlaser light incident obliquely is reflected by the retarder to and emitsin a form of circularly polarized light.

Further, when the direction of the fast axis of the retarder and thedirection of the linearly polarized light emitted from a semiconductorlaser source are in a specified relation, a substantially constant phasedifference, which does not substantially rely on an incident angle tothe retarder, is preferably obtainable. For example, when laser lighthaving a wavelength of 650 nm is used, the thin film of organic material201 is fixed to the fixing substrate by adjusting the direction of thefast axis of the thin film with respect to the direction of the linearpolarization of the incident light so that the ellipse ratio angleindicating a state of polarization of reflected light from the retarderis about 45° irrespective of an incident angle. With such arrangement,the emitted light reflected by the retarder produces a phase differenceof (2p−1)π/2 (p is a natural number) whereby linearly polarized lightcan be converted into circularly polarized light.

Here, the ellipse ratio angle means δ defined by tan α=±b/a (−45°≦α≦45°)where a is a long-axis amplitude intensity and b is a short-axisamplitude intensity of elliptically polarized light. In particular, byselecting the thin film of organic material having a predeterminedretardation value with respect to a specified wavelength, a reflectiontype retarder which provides a phase difference of 5π/2 with respect tolight having a wavelength band of 650 nm for DVD and a phase differenceof 2π with respect to light having a wavelength band of 790 nm for CD,can be formed.

Namely, when any of the retarders according to this embodiment isassembled in an optical head device using light having two differentwavelengths, light having a certain wavelength reflected by the retarderis rendered to be circularly polarized light and light having the otherwavelength is rendered to be linearly polarized light by adjusting theretardation value of the thin film of organic material. Accordingly, byarranging the retarder of this embodiment between the polarizingdiffraction element and the optical disk, there are obtainable thefeatures as follows. Namely, with respect to light having a wavelengthband of 650 nm, a high efficiency of utilization of light can beobtained because the retarder functions as a polarizing diffractionelement which allows to transmit in a going path and diffracts the lightin a returning path, and with respect to light having a wavelength bandof 790 nm, there is little influence of the birefringence of an opticaldisk because it transmits through the polarizing diffraction element ingoing and returning paths irrespective of a state of polarization.

[Embodiment 5]

FIG. 8 is a cross-sectional view showing the structure of an opticalelement according to a fifth embodiment of the present invention. A thinfilm of organic material 301 having birefringent properties is fixed byadhesives 302, 303 between a transparent substrate 305 having anaperture controlling function and a transparent substrate 304 having awavefront aberration correcting function, whereby an optical element 30having an aperture controlling function and a wavefront aberrationcorrecting function is formed. The wavefront aberration to be correctedincludes coma aberration, astigmatism, spherical aberration and so on.

For recording/reproducing CD, an optical disk having a thickness of 1.2mm, a semiconductor laser having a wavelength band of 790 nm and anobjective lens having NA (numerical aperture) of 0.45-0.5 is used, andfor recording/reproducing DVD, an optical disk having a thickness of 0.6nm, a semiconductor laser having a wavelength band of 650 nm and anobjective lens having NA of 0.6 is used. In a case of conductingrecording/reproducing of CD and DVD by using a single objective lens, anaperture limiting element for switching NA depending on light having awavelength for CD and light having a wavelength for DVD is required.Namely, the aperture limiting element is to transmit rectilinearly lighthaving wavelengths for both DVD and CD in a region of NA=0.45; totransmit rectilinearly only light for DVD in a peripheral regionsurrounding NA=0.45, and to prevent light for CD from transmitting.

Further, in a case of reading information in an optical disk of CD orDVD by using a same photodetector, an aberration correcting element isneeded because the spherical aberration caused by a difference inthickness of optical disks remains. In the optical element 30 accordingto this embodiment, the fixing substrates constituting the retarderpossess an aperture controlling function and an aberration correctingfunction which are required in a case of conductingrecording/reproducing information in optical disks having differentspecifications such as CD, DVD by using a single objective lens.Accordingly, miniaturization and high-performance can be realized in theoptical head device without increasing the number of structuralelements.

The transparent substrate 305 having an aperture controlling functioncomprises a transparent material such as glass, which has a first regionas a central portion and a second region surrounding the central portionwherein a periodic grating having a concave and convex shape in crosssection is formed directly in a thin film or in a surface of thetransparent substrate in the second region. By adjusting the amplitudeof phase caused by the concave and convex shape, it is possible totransmit or diffract a predetermined wavelength selectively.Specifically processing of the concave and convex shape is conducted sothat the phase difference caused by the concave and convex shape isintegral multiples of 2π with respect to a wavelength for DVD andnon-integral multiples of 2π with respect to a wavelength for CD,whereby light having a wavelength for DVD is allowed to transmit andlight having a wavelength for CD is diffracted. Thus, the aperturelimiting function is given to the transparent substrate 305 so thatlight is not converged to an information-recording portion in theoptical disks.

The transparent substrate 304 having a wavefront aberration correctingfunction comprises a glass substrate or the like, and it functions byforming a groove directly or in the thin film formed on a surface of thetransparent substrate in the first region wherein the groove has atwo-dimensionally distributed depth depending on a quantity ofaberration to be corrected so that a distribution of phase difference isproduced spatially. FIG. 8 shows the transparent substrates 304 and 305each having a different function. However, it is possible to formsimultaneously in the same surface of the same transparent substrate agroove for producing a wavefront aberration correcting function and aperiodical concave/convex-like diffraction grating for producing anaperture controlling function. In this case, it is possible to simplifysteps for processing.

[Embodiment 6]

FIG. 9 shows a cross-sectional view and a plan view showing thestructure of an optical element according to a sixth embodiment. FIG.9(a) shows an optical element 40A wherein a thin film of organicmaterial 401 having birefringent properties is fixed to a transparentsubstrate 405 having a diffracting function by an adhesive 403, and anantireflection film 406 is formed on the other surface of the thin filmof organic material 401. FIG. 9(b) shows, as another constitutionexample, an optical element 40B wherein a thin film of organic material401 having birefringent properties is fixed between a transparentsubstrate 405 having a diffracting function and a transparent substrate404 coated with an antireflection film 406 by adhesives 402, 403. FIG.9(c) is a plan view of the optical element of FIG. 9(c) and FIG. 9(b)viewed from an upper side.

The transparent substrate 405 having a diffracting function comprises atransparent material such as glass. A periodic grating having a concaveand convex shape in cross section or a hologram is formed directly or ina thin film formed on a surface of the transparent substrate. It ispreferable to provide an antireflection film to prevent a loss ofreflection of light at the interface between air and the transparentsubstrate 405 having a diffracting function.

With such multi-layered plate structure using the transparent substratesand the thin film of organic material, the retarder having a diffractingfunction can be formed. The optical element having a diffractingfunction according to this embodiment can be used as, for example, a3-beam generating element for detecting signals in a 3-beam method or adifferential push-pull method or the like. Since the diffraction elementused for a signal detecting system such as the 3-beam method or thedifferential push-pull method is located in the vicinity of incidentlight from a laser light source, it can be a small element. Further,incident light to the diffraction element becomes divergent light. Theoptical element of this embodiment provides a phase difference producingfunction while it is as small as the conventional diffraction element.Further, since the retarder having the thin film of organic material cansuppress the fluctuation of the retardation value with respect toincident divergent light, it is possible to stabilize a state ofpolarization of laser light.

In particular, since a thin film of organic material producing a phasedifference of π/2 is used for an optical head device mounting ahigh-power laser light source for recording CD-R or the like, thepolarized plane is rotated 90° by going and returning light between thesurface of the optical disk and the emitting point of laser lightthrough the thin film of organic material. Accordingly, a stable signalintensity of laser can be obtained without causing interference betweenthe oscillated light and the returning light, with the result thatstable recording of information in the optical disk can be realized.

[Embodiment 7]

In the optical head device according to a seventh embodiment of thepresent invention, a light source emitting laser light having two ormore different wavelengths is used, and a broadband retarder forcontrolling a state of phase of the laser light having two or moredifferent wavelengths is located between the light source and anobjective lens.

FIG. 10(a) is a cross-sectional view showing such broadband retarder 50.Two phase plates 501A and 501B each comprising a thin film of organicmaterial having birefringent properties are piled up between transparentsubstrates 504, 505 so that the respective optical axes are crossed, andeach of the members is fixed by using adhesives 502, 503 and 506. It hassuch construction that the retardation value of the phase plate 501A towhich laser light is first incident is larger than the retardation valueof the phase plate 501B to which the laser light is secondly incident,and the ratio of these retardation values is 1.8-2.2. With thisstructure, the broadband retarder functions as a quarter-wave platewhich provides a phase difference of about π/2 with respect to linearlypolarized laser light of any wavelength transmitting therethroughwhereby linearly polarized light is rendered to be substantiallycircularly polarized light.

In the following, description will be made as to a case that the numberof emitted laser light having different wavelengths is two. A typicalcombination of phase difference in the two phase plates constituting thebroadband retarder and a typical arrangement of the fast axis will bedescribed.

Assumed that retardation values produced by the piled phase plates 501Aand 501B wherein laser light is incident first to the phase plate 501Aand 501B secondly, are respectively R₁ and R₂. Further, definition ismade so that laser light having a shorter wavelength between twodifferent wavelengths is represented by λ_(L); laser light having alonger wavelength is represented by λ_(H), and a wavelength λ having arelation of λ_(L)≦λ≦λ_(H) is an intermediate wavelength between thetwo-wavelengths λ_(L) and λ_(H).

In this case, the wavelength λ is about two times of R₁ and about fourtimes of R₂; the ratio of the retardation values R₁/R₂ is 1.8-2.2, andthe optical axes of these two phase plates are crossed to each other. Itis preferable that the crossing angle is 45-75°. Then, theabove-mentioned effect, i.e. linearly polarized light being rendered tobe substantially circularly polarized light, is obtainable.

On the other hand, when the retardation value R1 of the phase plate towhich laser light in the two kinds of laser light is incident first islarger than the retardation value R2 of the phase plate to which laserlight is incident secondly; the ratio of the retardation values is 2,and each elliptical polarization produced when the two kinds of laserlight transmit through the broadband phase plate 50 is equal to eachother, namely, relations in formulae (2a) and (2b) are established, itis preferable because efficiency of utilization of light becomes equalwith respect to usable light having either wavelength. In the formulae,the retardation dispersion coefficient of the laser light having ashorter wavelength is represented by k_(L) and that of a longerwavelength is represented by k_(H).R ₁=(λ_(L)·λ_(H))/(2·(k _(L)·λ_(H) +k _(H)·λ_(L))  (2a)R ₂=(λ_(L)·λ_(H))/(4·(k _(L)·λ_(H) +k _(H)·_(L))  (2b)

Here, the retardation values of the first and second phase platesgenerally have wavelength dependence. When A, B and C indicaterespectively dispersion coefficients which depend on materials, it isrepresented by R=A+B/(λ−C) approximately. Further, the dispersioncoefficients k_(L) and k_(H) are defined as in formulae (3a) and (3b)respectively. Here, a numerical value 589 means the wavelength of aD-line of sodium used as the standard wavelength for measuring theretardation values.k _(L) =R(λ_(L))/R(589)={A+B/(λ_(L) −C)}/{A+B/589−C}  (3a)k _(H) =R(λ_(H))/R(589)={A+B/(λ_(H) −C)}/A+B/589−C  (3b)

The two kinds of laser light incident into the broadband retarder havethe same direction of linearly polarized light. With respect to thedirection of the linearly polarized light, the direction of the fastaxis in optical axes of one phase plate indicates an angle of 10-20°,and the direction of the fast axis in optical axes of the other phaseplate indicates an angle of 70-80°. At these angles, the wavelengthdependence of the ellipse ratio angle is small, namely, the ellipseratio angle does not largely change depending on wavelengths. The effectof preventing such change is not influenced even in case that the anglein the first phase plate to which light is incident first is 10-20° andthe angle in the second phase plate is 70-80, or even in a case that theangle in the first phase plate is 70-80° and the angle in the later oneis 10-20°.

In the following, detailed description will be made as to these angles.Assumed that angles formed between the direction of the linearpolarization of incident laser light and the respective lead axes of thetwo phase plates are respectively θ₁ and θ₂ in the order of phase platesto which laser light is successively incident. When the wavelength λ isabout two times of R1 and about 4 times of R₂, namely, relations ofR₁=λ/2 and R₂=λ/4 establish substantially, the ellipse ratio angles canbe calculated as the functions of θ₁ and θ₂.

FIG. 11 shows the ellipse ratio angles wherein a ridge formed by twoinclined planes satisfies the condition that the ellipse ratio angle issubstantially 90°. FIG. 11(a) is a graph in which the ridge isrepresented by Formula (4a) where θ₂ is larger than θ₁, and FIG. 11(b)is a graph in which the ridge is represented by Formula (4b) where θ₁ islarger than θ₂. Under the conditions of Formulae (4a) and (4b), thebroadband phase plate functions substantially as a quarter-wave plate.θ₂=2×θ₁+π/4  (4a)θ₁=2×θ₂+π/4  (4a)

If the wavelength of usable laser light is completely in coincidencewith a designed wavelength, and θ₁ and θ₂ are in an angular relation tosatisfy Formula (4a) or Formula (4b), completely circularly polarizedlight of transmitted light is obtainable. However, when the wavelengthdeviates slightly from the designed wavelength, and when relationsdescribed in Formulae (5a), (5b) are established even though θ₁ and θ₂have a combination of angle satisfying the relation of Formula (4a) orFormula (4b), the transmitted light becomes closest to circularlypolarized light. The unit of the angle is degree in any relationalformula.(θ₁, θ₂)=(15, 75)  (5a)(θ₁, θ₂)=(75, 15)  (5b)

From the above, it is in particular preferable that θ₁ and θ₂ haveangles given by Formula (5a) or Formula (5b), or they have angles in thevicinity of these angles in a range of ±5°. For example, with respect toFormula (5a), (θ₁, θ₂)=(10-20, 70-80) is given as described above.

FIG. 12 shows the wavelength dependence of transmitting light to theellipse ratio angle in a case of (θ₁, θ₂)=(15, 75) and (10, 65) amongvarious combinations of the angles of θ₁ and θ₂ satisfying Formula (4a).It is found in FIG. 12 that completely circularly polarized light can beobtained because the ellipse ratio angle is 90° in a designed wavelength(718 nm), and the change of the wavelength dependence in (θ₁, θ₂)=(15,75) (solid line) is smaller than the change of the wavelength dependencein (θ₁, θ₂)=(10, 65) (broken line).

Further, in FIG. 12, as the wavelength of laser light used is apart fromthe designed wavelength, the ellipse ratio angle shifts from 90° wherebytransmitted light becomes a form of elliptically polarized light. In theelliptically polarized light, it is estimated that with respect to acombination of, for example, a wavelength band of 400 nm used for anoptical disk of high density recording and a wavelength band of 790 nmused for CD, polarized light is largely changed from circularlypolarized light even when any wavelength is used.

The change from the circularly polarized light, when a polarizingdiffraction element is used as a beam splitter for instance,deteriorates the diffraction characteristics in a returning path tothereby reduce a reproduced signal intensity of reflected light from theoptical disk. Accordingly, such change is problematic in the opticalhead device. The wavelength usable actually in the optical head devicedoes not cover the entire wavelength band region includingtwo-wavelengths from the laser light source, but covers only two kindsof wavelength, and therefore, only the polarizing performance withrespect to the two-wavelengths is in question. Accordingly, in thepresent invention, the relation of the directions of lead axes of thetwo phase plates was determined so that the ellipse ratio angle becamethe maximum with respect to the two-wavelengths used.

The oblique solid line in FIG. 13(a) indicates Formula (4a) and theoblique solid line in FIG. 13(b) indicates Formula (4b). As shown byblack circles in FIG. 13(a) and FIG. 13(b), θ₁ and θ₂ functioning as acomplete quarter-wave plate for two usable wavelengths satisfy therelation of Formula (6a) (FIG. 13(a)) or Formula (6b) (FIG. 13(b)).(θ₁, θ₂)=(15+a, 75−a)  (6a)(θ₁, θ₂)=(75−a, 15+a)  (6b)

where a is a positive coefficient determined by the interval between twousable wavelengths. Table 1 shows θ₁ and θ₂ on which Formula (6b) isestablished under a condition 1 (a=0) and a condition 2 (a=3.2), and R₁and R₂ at these angles.

TABLE 1 θ₁ θ₂ a (degree) (degree) (degree) R₁ (nm) R₂ (nm) Condition 175.0 15.0 0.0 260 130 Condition 2 71.8 18.2 3.2 260 130

FIG. 14 shows the wavelength dependence of the ellipse ratio angle oflight transmitting the broadband retarder in cases of a=0 (broken line)and a=3.2 (solid line). In the determination of an appropriate rotationangle of 3.2° based on set center value of (θ₁, θ₂)=(75, 15), theellipse ratio angle to a used wavelength band of 425 nm and a usedwavelength band of 790 nm is remarkably improved, and the ellipse ratioangle on the two used wavelength bands becomes 90° whereby completelycircularly polarized light is formed. Similarly, completely circularlypolarized light can be obtained by determining an appropriate rotationangle even in a case of said center value of (θ₁, θ₂)=(15, 75).

As described above, the broadband retarder can fully have function as aquarter-wave plate with respect to two-wavelengths wherein phase plateshaving retardation values satisfying Formulae (2a), (2b) on optionaltwo-wavelengths are used, and the optimum a value is set to determinethe directions of the lead axes of the two phase plates in thisembodiment. Here, the value of a is 0.2 in a case of, for example, usingtwo-wavelength bands of 660 nm and 790 nm, and 3.2 in a case of using awavelength band of 425 nm and a wavelength band of 790 nm.

Accordingly, it is extremely preferable to determine the value of a sothat the broadband retarder completely functions as a quarter-wave platewithin a range of establishing Formula (6a) or Formula (6b) with respectto the two specified wavelengths of light, namely, the ellipse ratioangle becomes 90°.

It is preferable that the two phase plates, the fixing substrates andthe adhesives in the broadband retarder are constituted by usingmaterials as described in the first embodiment. It is preferable thatfor the purpose of reducing the wavefront aberration of the broadbandretarder and improve the temperature characteristics and reliability,the adhesives for bonding should be thin as possible. It is inparticular preferable that the thickness of adhesive layers is 10 μm orless.

The broadband retarder according to this embodiment can be used solely.However, when it is unified by laminating on another optical elementused in the optical head device, it is possible to minimize the numberof elements, simplify assembling work for the optical head device, andmake the device small. Accordingly, it is preferable that the broadbandretarder is unified with at least one optical element to change theoptical characteristics of laser light. Specifically, the opticalelements described in the third to sixth embodiments are mentioned.

The broadband retarder of this embodiment provides in particularremarkable effect when it is used in an optical head device having anoptical element utilizing a difference of polarization properties, andis suitable as a part for an optical head device used forrecording/reproducing information to achieve miniaturization andweight-reduction.

In an optical head device shown in FIG. 15, an optical element 51 formedby unifying a polarizing hologram 51B with a quarter-wave plate 51A isattached to an objective lens 515 mounted on an actuator 517. Thequarter-wave plate 51A used here is the broadband retardation plate ofthe present invention. Laser light emitted respectively from asemiconductor laser source 511A having a wavelength band of 660 nm and asemiconductor laser source 511B having a wavelength band of 790 nm aremade parallel by collimater lenses 513A and 513B respectively, andtransmit the polarizing hologram 51B and the quarter-wave plate 51Athrough a beam splitter prism 514 to be converged on the informationrecording surface of an optical disk 516 by means of the objective lens515.

Reflected light including information recorded in the optical disk 516propagate in the reverse direction through respective paths. Returninglight transmitting through or reflected by the beam splitter prism 514pass through the collimater lenses 513A, 513B respectively, and detectedby a photodiode 512A for a wavelength band of 660 nm and a photodiode512B for a wavelength band of 790 nm respectively.

In the structure shown in FIG. 15, when a polarizing hologram which isoptimized to laser light having either wavelength between thetwo-wavelengths or which is optimized to laser light having a wavelengthof intermediate value between the two-wavelengths is used, a hightransmittance property is exhibited in a going path, and a reduction ofefficiency in question does not take place in a returning path withrespect to light having either wavelength.

[Embodiment 8]

FIG. 16 is a cross-sectional view showing the structure of a retarder 60according to an eighth embodiment of the present invention. The retarder60 is formed by piling phase plates 601A and 601B each comprising a thinfilm of organic material having birefringent properties with respectiveoptical axes being crossed, between transparent substrates 604 and 605,each of the above-members being fixed by using adhesives 602, 603 and606, in the same manner as the structure of the broadband retarder ofthe seventh embodiment. It is preferable that the two phase plates, thefixing substrates and the adhesives in the retarder are materialsdescribed in the first embodiment.

The phase plates 601A and 601B have the same retardation value, and thephase plates 601A and 601B are piled up each other so that therespective optical axes are crossed. When linearly polarized lighthaving wavelengths of λ₁ and λ₂ (λ₁<λ₂) enter into the retarder 60, theplanes of polarized light of respective linearly polarized light rotateby the same angle. The retardation values R_(d) of the respective phaseplates have the same value, and R_(d)=λ/2 in linearly polarized lighthaving a wavelength λ in a relation of λ₁≦λ≦λ₂.

In a case that a retarder is to be formed in consideration of lighthaving a wavelength band of λ₁=660 nm for DVD and light having awavelength band of λ₂=790 nm for CD, and phase plates having aretardation value in a range of R_(d)=330-395 nm are used for example,good linearity maintaining properties and optical rotation performanceon incident linearly polarized light can preferably obtained. Theabove-mentioned performance becomes maximum when R_(d) takes anintermediate value of the above-mentioned range, i.e. about 362 nm.Further, the above-mentioned performance can be recognized even when itis smaller the R_(d)=330 nm, e.g., about 310 nm.

The rotation angles of the two planes of polarized light of linearlypolarized light produced by the retarder are the same. A rotation angleφ is given in a relation of φ=180°−2θ wherein an angle θ is formedbetween the optical axes of the phase plates 601A and 601B. Accordingly,by adjusting the angle θ formed between the optical axis of the phaseplate 601A and the optical axis of the phase plate 601B, a desiredrotation angle φ can be obtained. In the retarder constituted as above,the planes of polarized light can be rotated by the same angle whilekeeping the linearity with respect to two different kinds of linearlypolarized light even though their wavelengths are different.

FIG. 17 is a cross-sectional view showing an constitution example of anoptical head device installing the retarder according to the eighthembodiment of the present invention. Two kinds of linearly polarizedlight having different wavelengths emitted from semiconductor lasersources 611A and 611B are reflected by beam splitters 614A and 614Brespectively, and then, transmit through the retarder 60 to be convergedon the information recording surface of an optical disk 616 by means ofa collimater lens 613 and an objective lens 615. After the two kinds oflinearly polarized light having different wavelengths have passedthrough the retarder 60, their planes of polarized light are rotated byan angle φ, and then, the two kinds of light are reflected by theinformation recording surface of the optical disk 616. The reflectedlight in a returning path are converged by the objective lens 615 andthe collimater lens 613, and again transmit through the retarder 60. Thelight in the returning path transmitting through the retarder, aftertheir planes of polarized light are made in coincidence with the planesof polarized light in the going path, are converged on a photodetector612.

In the optical head device using the retarder of the present invention,a fluctuation in the intensity of signal light reflected by the opticaldisk, caused by a distribution of the birefringence remaining in theoptical disk or the polarization dependence can be reduced, and stablerecording and reproducing of information can be obtained.

[Embodiment 9]

FIG. 18 shows a cross-sectional view of the retarder 70 according to aninth embodiment of the present invention. A phase plate 701 comprisinga thin film of organic material and fixing substrates 704, 705, and theyare fixed by using adhesives 702, 703. The retarder has a structureusing the same materials as described in the first embodiment.

The retarder 70 is constructed as follows. Assumed that laser lighthaving two different wavelengths λ₁ and λ₂ emitted from light sources,are incident, as linearly polarized light having the planes of polarizedlight which are parallel to each other, into the retarder 70. Theretardation value of the phase plate is adjusted so that a phasedifference of 2π(m₁−½) is produced when light having a wavelength λ₁transmits through the phase plate 701; a phase difference of 2πm₂ isproduced when light having the other wavelength λ₂ transmits through thephase plate, and the planes of polarized light of two kinds of linearlypolarized light are crossed to each other where m₁ and m₂ are naturalnumbers, and the linearly polarized light of wavelengths λ₁, λ₂ areincident into the phase plate with an inclination in a polarizationdirection of 45° with respect to the direction of the fast axis (notshown) of the phase plate.

Description will be made as to a case that for example, the wavelengthof laser light having a wavelength band of 660 nm for DVD is representedby λ₁ and the wavelength of laser light having a wavelength band of 790nm for CD is represented by λ₂. A phase plate made of polycarbonatehaving a retardation value of 5λ₁/2(m₁=3) to linearly polarized lighthaving a wavelength of λ₁ is arranged so that the fast axis is inclined45° with respect to the direction of incident linearly polarized light.Then, when the linearly polarized light of a wavelength of λ₁ transmitsthrough the phase plate, the polarization plane rotates 90°. On theother hand, the retardation value of the phase plate to the linearlypolarized light of wavelength λ₂ corresponds to 2λ₂(m₂=2), and thepolarization plane of the linearly polarized light of wavelength λ₂transmitting through the phase plate does not rotate. Namely, thelinearly polarized light having wavelengths of λ₁ and λ₂ whose theplanes of polarized light are parallel to each other, become linearlypolarized light whose the planes of polarized light crossperpendicularly after the light have transmitted through the phase plate701.

Further, a case of using a wavelength λ₁ of 800 nm and a wavelength λ₂of 400 nm, and a phase plate made of polycarbonate having a retardationvalue of λ₁/2(m₁=1) to light having a wavelength of λ₁ which is arrangedso that the fast axis is inclined 45° with respect to the direction ofincident linearly polarized light is considered. When the linearlypolarized light of wavelength λ₁ transmits through the phase plate, thepolarization plane rotates 90°. On the other hand, the retardation valueof the phase plate to the linearly polarized light of wavelength λ₂corresponds to λ₂(m₂=1), and the polarization plane of the linearlypolarized light of wavelength λ₂ transmitting through the phase platedoes not rotate. Namely, even in this case, the linearly polarized lighthaving wavelengths λ₁ and λ₂ whose the planes of polarized light areparallel to each other, become linearly polarized light whose the planesof polarized light cross perpendicularly after they have transmittedthrough the phase plate 701.

Here, values taken by the natural numbers m₁ and m₂ are preferably about3 or less. If m₁ and m₂ are about 3 or more, an elliptical polarizationis produced because a change in the retardation value to a fluctuationof the laser light wavelength λ₁ or λ₂ becomes large. It is difficult tomanufacture a phase plate having a large retardation value with use of asingle thin film of organic material. If a plurality of thin films islaminated in order to produce such phase plate, the number of steps formanufacturing will increase.

The retarder 70 constructed as mentioned above has function to convertlinearly polarized light having two different wavelengths whose theplanes of polarized light are parallel to each other into linearlypolarized light whose the planes of polarized light crossperpendicularly. Accordingly, light having two different wavelengths canbe separated by providing different planes of polarized light.

[Embodiment 10]

FIG. 19 is a cross-sectional view of an optical element 71 according toa tenth embodiment of the present invention. A phase plate 701comprising a thin film of organic material fixed to a transparentsubstrate 704 by an adhesive 702, and a transparent substrate 705 inwhich a saw-teeth like grating 706 is formed is fixed to the phase plate701 by a filling adhesive 703 so that the saw-teeth like grating 706opposes the phase plate 701. The phase plate 701 has the same structureand function as the retarder according to the ninth embodiment whereinlinearly polarized light of two different wavelengths whose the planesof polarized light are parallel to each other transmit through the phaseplate with an inclination of 45° in a polarization direction withrespect to a direction of the fast axis of the phase plate, whereby theplanes of polarized light of light having two different wavelengthscross perpendicularly each other. The saw-teeth like grating 706functions as a polarizing diffraction grating which diffracts either oneof the linearly polarized light of two different wavelengths whose theplanes of polarized light cross perpendicularly each other and does notdiffract the other.

Here, the saw-teeth like grating 706 as a polarizing diffraction gratingfunctions so that when two kinds of light of different wavelengths areincident with their optical axes directing certain angles, only one kindof light is diffracted so that the optical axis of the one kind of lightis aligned with the optical axis of the other light as shown in FIGS.19(a) and 19(b). The saw-teeth like grating 706 is made of abirefringent material such as polymerized liquid crystal having anordinary refractive index n_(o) and an extraordinary refractive indexn_(e), and is prepared by the following steps. The before-mentionedpolymerized liquid crystal is coated on the transparent substrate 705 ina predetermined thickness. Liquid crystal molecules are made aligned sothat an orientated vector of the liquid crystal molecules in a directionof extraordinary refractive index n_(o) of the polymerized liquidcrystal is aligned in a specified direction in the surface. After theliquid crystal molecules have been polymerized by irradiating light froma photo-polymerizing light source, a saw-teeth like grating is formed bytechniques of photolithography and etching. It is preferable that thesaw-teeth like grating have a right triangle shape in cross section.However, it may be a step-like shape as shown in FIG. 19. In the case ofthe step-like shape, the larger the number of steps is, the greaterefficiency of utilization of light is obtainable. In forming four ormore steps, the first-order light of 70% were more can preferably beutilized.

The filling adhesive 703 is a transparent adhesive having a uniformrefractive index n_(s), and a material for the adhesive is selected sothat the refractive index n_(s) is substantially equal to the ordinaryrefractive index n_(o) of polymerized liquid crystal, for example. Thefilling adhesive can appropriately be selected among the adhesivesdescribed in the first embodiment. The optical element 71 having theabove-mentioned structure functions to convert the two kinds of linearlypolarized light of different wavelengths whose the planes of polarizedlight are in parallel to each other into linearly polarized light whosethe planes of polarized light cross perpendicularly each other;separates the two kinds of light of different wavelengths, and functionsas a diffraction grating with respect to linearly polarized light of awavelength λ₂(λ₁≠λ₂) although it does not function as a diffractiongrating with respect to linearly polarized light of a wavelength λ₁, forinstance.

As another example of this embodiment, a polarizing diffraction gratingfor generating three beams for tracking in an optical head device may beused instead of the above-mentioned saw-teeth like grating 706, whichfunctions as a diffraction grating to linearly polarized light of awavelength λ₁ and does not function as a diffraction grating to linearlypolarized light of a wavelength of λ₂.

As a separate example of this embodiment, an optical element comprisinga transparent substrate to which a function to produce a spatialdistribution of phase to the transmission wavefront is added, may beused instead of the above-mentioned saw-teeth like grating 706. Withthis, the wavefront of incident linearly polarized light can becontrolled. For instance, when the above-mentioned polymerized liquidcrystal on the transparent substrate is processed so that the layerthickness is spatially distributed, there is no change in a distributionof phase with respect to incident ordinary polarized light of awavelength λ₁, but a distribution of phase corresponding to adistribution of layer thickness is produced with respect to incidentextraordinary polarized light of a wavelength λ₂. When such retarder ismounted on an optical head device, aberration remaining by the cause ofthe difference of wavelength can be corrected. For example, it iseffective as an compatible element between DVD and CD.

[Embodiment 11]

FIG. 20 is a cross-sectional view of an optical head device according toan eleventh embodiment of the present invention. The optical head devicecomprises a two-wavelength semiconductor laser source 711 having twolight emitting points 711A and 711B disposed with an interval togenerate linearly polarized light having a wavelength λ₁ and awavelength λ₂(λ₁≠λ₂), whose the planes of polarized light are inparallel to each other, an optical element 71 according to the tenthembodiment, a beam splitter 714, a collimater lens 713, an objectivelens 715 and a photodetector 712, to perform the recording andreproducing of an optical disk 716.

When linearly polarized light of a wavelength λ₁ is incident into theoptical element 71, the polarization plane of the linearly polarizedlight rotates 90° by the phase plate constituting the optical element71, and the linearly polarized light becomes ordinary light to thepolymerized liquid crystal constituting a saw-teeth like grating.Accordingly, the linearly polarized light transmits straightly becausethe ordinary refractive index n_(o) of the saw-teeth like grating issubstantially equal to the refractive index n_(s) of the fillingadhesive (n_(o)=n_(s)). On the other hand, since the light emittingpoint which emits the linearly polarized light of a wavelength λ₂incident into the phase plate is separated from the light emitting pointwhich emits the linearly polarized light of a wavelength λ₁, the opticalaxis of the linearly polarized light having a wavelength λ₁ is inclinedwith respect to the optical axis of the linearly polarized light of awavelength λ₂, and accordingly, the polarization plane of the light doesnot rotate even when it transmits through the phase plate whereby thelinearly polarized light of a wavelength λ₂ becomes extraordinary lightto the polymerized liquid crystal constituting the saw-teeth likegrating. Accordingly, the saw-teeth like grating generates the +first-order diffraction light because the extraordinary refractive indexn_(e) of the saw-teeth like grating is different from the refractiveindex n_(o) of the filling adhesive, and by adjusting the pitch ofgrating, the + first-order diffraction light having a wavelength λ₂ isemitted by being diffracted into the same axial direction as the opticalaxis of the light of a wavelength λ₁.

Namely, the optical head device mounting thereon the optical element 71of this embodiment can make the optical axes of linearly polarized lighthaving wavelengths λ₁ and λ₂ (λ₁≠λ₂) emitted from the two-wavelengthsemiconductor laser source coincident with each other. Accordingly, theother optical elements constituting the optical head device performexcellent optical characteristics because the optical axes of the twokinds of linearly polarized light having different wavelengths arecoincident with each other, and recording and reproducing of informationin an optical disk can stably be conducted. Further, since detection ofsignal light having wavelengths λ₁ and λ₂ can be conducted by the samephotodetector, the structure of the optical head device can besimplified; the number of assembling steps can be reduced, andminiaturization and weight-reduction of the device can be realized.

Examples of the present invention will be described hereinbelow.

EXAMPLE 1

A retarder 1A as shown in FIG. 1 in cross section was prepared.Uniaxially stretched (alicyclic) polyolefin (linear expansioncoefficient E₁=6.1×10⁻⁶/° C., glass transition temperature T₁=171° C.)was used as the thin film of organic material 101 having birefringentproperties, an acrylic type adhesive (linear expansion coefficientE₂=1.1×10⁻⁴/° C.) was used as the adhesives 102, 103 and glasssubstrates (linear expansion coefficient E₃=95×10⁻⁷/° C.) were used asthe fixing substrates 104, 105. The retardation value of the thin filmof organic material was 170 nm.

As a result of using the retarder of Example 1 in an optical headdevice, there was no change of phase difference and the direction ofretardation axis in the retarder and no disturbance of the wavefront oflaser light passing through the retarder caused by a deformation of thethin film of organic material even when there was a fluctuation of thewavelength in a semiconductor laser source due to a temperature change.Accordingly, there was no disadvantage such as a reduction of detectedlight quantity in the photodetector, whereby good reproducing signalsfor information in the optical disk could be obtained. Further, theusable temperature was 150° C. or more; desired characteristics could beobtained even in a high temperature region, and there was no trouble inuse.

COMPARATIVE EXAMPLE 1

A retarder was prepared as a Comparative Example of the retarder shownin FIG. 1. Uniaxially stretched polyester (linear expansion coefficientE₁=6×10⁻⁵/° C.) was used as the thin film of organic material having aphase-difference producing function, an epoxy type adhesive (linearexpansion coefficient E₂=5×10⁻⁵/° C.) was used as the adhesives andglass substrates (linear expansion coefficient E₃=95×10⁻⁷/° C.) wereused as the fixing substrates. The retardation value of the retarder was330 nm.

In this Comparative Example, the linear expansion coefficients E₁, E₂and E₃ did not satisfy the relation of the before-mentioned Formula (1).Accordingly, as a result of using the retarder in an optical headdevice, a change of phase difference was caused in the retarder due to atemperature change and a disadvantage such as a reduction of detectedlight quantity in the photodetector was resulted. Further, at atemperature of 130° C. or more, the thin film of organic material peeledoff from the fixing plates to cause the destruction of the retarder.

EXAMPLE 2

A polarizing diffraction type optical element 10 was prepared byunifying a retarder 1B with a polarizing diffraction element 2 as shownin FIG. 4. Uniaxially stretched polycarbonate (linear expansioncoefficient E₁=6.2×10⁻⁶/° C.) was used as the thin film of organicmaterial 101 having birefringent properties, a polyester type adhesive(linear expansion coefficient E₂=1.2×10⁻⁴/° C.) was used as theadhesives 102, 103, and glass substrates (linear expansion coefficientE₃=95×10⁻⁷/° C.) were used as the fixing substrates 104, 105.

The retardation value of the thin film of organic material 101 as asingle body was 812 nm and the thickness was 45 μm. Since the glasssubstrates 104, 105 and the adhesives 102, 103 had no birefringentproperties, hence, they had no phase-difference producing function, theretardation value of the optical element 10 was 812 nm which was thesame as that of the thin film of organic material 101 as a single body.Further, the transmission wavefront aberration of the optical element 10was 0.006 λrms and extremely excellent wavefront characteristics couldbe obtained.

The optical element 10 was installed in the optical head device shown inFIG. 21. The optical element 10 formed by unifying a polarizingdiffraction element 2 with a retarder 1B having a retardation value of5λ/4 was disposed in an optical path between a semiconductor lasersource 111 having an oscillated wavelength λ in a wavelength band of 650nm and an objective lens 115, and such optical system was disposed so asto oppose to an optical disk 116. Further, a photodetector 112 wasdisposed in an optical path in a diffraction direction of the polarizingdiffraction element 2. While light emitted from the semiconductor lasersource 111 transmits through the polarizing diffraction element 2 andthe retarder 1B, linearly polarized light is converted into circularlypolarized light. The circularly polarized light is converged on theoptical disk 116 by means of the objective lens 115, and reflectedtherefrom. The reflected light from the optical disk 116 transmitsthrough the objective lens 115 and the retarder 1B, whereby thecircularly polarized light is converted into linearly polarized light.The linearly polarized light is diffracted by the polarizing diffractionelement 2. Then, the diffraction light diffracted by the polarizingdiffraction element is received by the photodetector 112.

As a result of using the optical element 10 as a polarizing diffractiontype retarder of Example 2 in the optical head device, the adhesivesabsorbed thermal expansions caused in the thin film of organic materialand the fixing substrates of the retarder even when there was atemperature change, whereby a deformation of the retarder was preventedand a predetermined phase difference could always be obtained stably.With this, a change of phase difference, a change of the direction ofretardation axis or a disturbance of the wavefront of laser lightpassing through the retarder did not occur. Accordingly, a disadvantagesuch as a reduction of detected light quantity in the photodetector didnot occur and excellent reproducing signals for the optical disk couldbe obtained.

EXAMPLE 3

In this example, a reflection type retarder 20A as shown in FIG. 5 wasprepared. The retarder 20A was prepared by using uniaxially stretchedpolycarbonate (linear expansion coefficient E₁=6.2×10⁻⁶/° C.) as thethin film of organic material 201 having a phase-difference producingfunction, a polyester type adhesive (linear expansion coefficientE₂=1.2×10⁻⁴/° C.) as the adhesive 202 and a glass substrate (linearexpansion coefficient E₃=95×10⁻⁷/° C.) with vapor-deposited aluminum ina thickness of 100 nm as the fixing substrate 204 having a reflectingfunction. The thickness of the thin film of organic material was 45 μm,which provided a retardation value of 408 nm (i.e., a phase differenceof 5π/4) to light having a wavelength band of 652 nm which enteredperpendicularly into and transmitted through the thin film of organicmaterial, and a retardation value of 816 nm (i.e., a phase difference of5π/2) which was double in value in a case of being reflected by theretarder with the thin film of organic material.

As shown in FIG. 22, when light emitted from a semiconductor lasersource 211 having a wavelength band of 652 nm transmits through apolarizer 221, the light becomes linearly polarized light which is inparallel to an X axis and is incident into the retarder 20A. Here, anincident angle θ is an angle formed between a normal line set up on theretarder 20A and the X axis. In this case, adjustment was made to theretarder 20A so that the ellipse ratio angle was about 45°, and the thinfilm of organic material was attached to the fixing substrate 204 at analuminum film side by using the adhesive 202 so that the direction φ ofa slow axis of the thin film was at an angle of 42° to the X axis in thecoordinate system shown in FIG. 22.

In the retarder thus prepared, the incident angle dependence of a phasedifference of reflected light was examined. As a result, light reflectedby the retarder at an incident angle of θ=20-50° produced a retardationof 806-824 nm. It shifted as slight as about 9 nm at the maximum fromthe retardation value of 816 nm in a case of perpendicularly incident,and the phase difference of the reflected light was about 5π/2 in astate of circularly polarized light.

Further, the incident angle dependence of a phase difference wasexamined similarly by using a semiconductor laser source having awavelength band of 780 nm. As a result, light reflected by the retarderat an incident angle of θ=20-50° produced a retardation of 779-797 nm.It shifted as slight as about 9 nm at the maximum from the retardationvalue of 788 nm in a case of perpendicularly incident, and the phasedifference of the reflected light was about 2π in a state of linearlypolarized light.

Accordingly, the retarder 20A having such reflecting function couldconvert linearly polarized light having a wavelength band of 650 nmentering in a wide angular range of θ=20-50° into reflected light ofsubstantially uniform circularly polarized light; rendered linearlypolarized light having a wavelength band of 780 nm to be reflected lightof the similar linearly polarized light, and could reduce the incidentangle dependence of a phase difference to reflected light having eachwavelength band.

EXAMPLE 4

An optical element 30 as shown in FIG. 8 in which an aperturecontrolling function and a wavefront aberration correcting function wereadded to the retarder was prepared. As the thin film of organic material301 constituting the retarder, such one for producing a retardationvalue of λ/4 to an averaged wavelength λ between a wavelength band of650 nm and a wavelength band of 780 nm was used (uniaxially stretchedpolycarbonate, linear expansion coefficient E₁=6.2×10⁻⁶/° C.). Further,a polyester type adhesive (linear expansion coefficient E₂=1.2×10⁻⁴/°C.) was used as the adhesives 302, 303.

A quartz substrate (linear expansion coefficient E₃=5×10⁻⁷/° C.) wasused as the transparent substrate 305 having an aperture controllingfunction, and a periodic linear grating having a concave and convexshape in cross section was formed directly on the substrate in aperipheral region surrounding a central region including the opticalaxis of the transparent substrate by combining a photolithography methodand an etching method. The depth of concave and convex portions was 2.9μm so that light having a wavelength of 650 nm for DVD passed throughthe concave and convex portions without any diffraction, but the almostamount of incident light having a wavelength band of 790 nm for CD didnot passed through.

A quartz substrate was also used as the transparent substrate 304 havinga wavefront aberration correcting function, wherein a ringed belt-likegroove was formed directly in a central region including the opticalaxis of the transparent substrate by combining a photolithography methodand an etching method. The optical element 30 thus prepared wasassembled in an optical head device as shown in FIG. 15. In the opticalhead device of FIG. 15, a semiconductor laser source 511A having awavelength band of 650 nm and a semiconductor laser source 511B having awavelength band of 780 nm were used as the light sources. Light emittedfrom each of the semiconductor laser sources transmit through collimaterlenses 513A, 513B respectively and are introduced into a beam splitter514 in which the optical axes are agreed with each other. Then, thelight transmit through a polarizing diffraction element 51B.

As the polarizing diffraction element 51B, a polarizing hologram whereina lattice-like concave and convex portion was formed in a thin film ofpolymerized liquid crystal having birefringent properties, and theconcave and convex portion of the thin film of polymerized liquidcrystal was filled with an optically isotropic medium havingsubstantially the same refractive index as the ordinary refractive indexof the polymerized liquid crystal, was used. In this polarizinghologram, the diffraction efficiency varies depending on polarizationdirections of incident light. In a going path from the semiconductorlaser source to the optical disk, a polarization direction of hightransmittance is utilized, and in a returning path from the optical diskto the photodetector, a polarization direction of high diffractionefficiency is utilized.

Light transmitting through the polarizing diffraction element 51Btransmits through the optical element 30 having both an aperturecontrolling function and a wavefront aberration correcting function ofthis example; is converged on the recording surface of an optical disk516 by means of an objective lens 515, and is reflected by the opticaldisk. Reflected light from the optical disk is deflected slightly in itsoptical path by the optical element 30 and the polarizing diffractionelement 51B, and is introduced into respective photodetectors 512A, 512Bthrough a beam splitter 514.

In this example, by assembling the optical element 30 having an aperturecontrolling function and a wavefront aberration correcting function, asthe optical element of the present invention in the optical head device,the number of parts constituting the optical head device could bereduced to thereby realize miniaturization. Further, since the opticalelement performed optimum aperture control at the time of reproducing CDand was capable of correcting the wavefront aberration, the aberrationcould be reduced, and good reproducing signals could be obtained.

EXAMPLE 5

This example will be described with reference to FIG. 10(a) and FIG.10(b). A broadband phase plate was prepared by laminated thin films oforganic material 501A, 501B on fixing substrates 504, 505 made of glasshaving a thickness of 0.5 mm and fixing them by using UV curing typeadhesives 502, 503. The thin film of organic material 501A is a phaseplate having birefringent properties wherein the retardation valueobtained by stretching polycarbonate is 260 nm and the thickness is 30μm. The thin film of organic material 501B is a phase plate havingbirefringent properties wherein the retardation value obtained bystretching polycarbonate is 130 nm and the thickness is 30 μm. Thethickness of each adhesive layer was about 5 μm.

Materials for constituting the broadband phase plate, i.e., for the thinfilms of organic material 501A, 501B, the adhesives 502, 503 and thefixing substrates 504, 505, are the same as those of Example 1.Accordingly, the linear expansion coefficients of the respectivematerials are the same as those in Example 1. The retardation values andthe optical axis directions were measured by using semiconductor laserlight having a wavelength band of 660 nm. With measured optical axes,the thin films of organic materials were arranged so that the fast axisdirection 509A among optical axes of the thin film of organic material501A is at an angle of about 54° with respect to the fast axis direction509B among optical axes of the thin film of organic material 501B.

In measuring the angles, a counterclockwise direction observed from aside of the thin film of organic material 501B was taken as positive (+)when it is assumed that the thin film of organic material 501B islocated above the thin film of organic material 501A. By taking adirection of −18° (the sign concerning the angular direction is definedabove) with respect to the fast axis direction 509B of the thin film oforganic material 501B as standard, the broadband phase plate was cut tohave outer dimensions of 5 mm×5 mm by a dicing saw to obtain a broadbandphase plate element 50. The fast axis direction 509 of the broadbandphase plate was defined as an intermediate direction between respectivefast axis directions of two laminated thin films of organic material.

The wavelength of emitted light from a semiconductor laser source havinga wavelength band of 860 nm was taken as standard, and the ellipse ratioangle of the broadband phase plate element 50 was measured by using thesecond harmonic laser light having a wavelength band of 430 nm generatedby using a non-linear optical crystal KNbO₃ and emitted light from asemiconductor laser source having a wavelength band of 789 nm. Thelinear polarization direction 508 of the above-mentioned laser lighthaving two kinds of wavelength was at an angle of −45° with respect tothe fast axis direction 509 of the broadband phase plate element. Thelight was introduced from a side of the fixing substrate 504 of glass.The measured ellipse ratio angle was about 86° to the laser light havingwavelength band of 430 nm and about 88° to the wavelength band of 789nm. Both were close to 90°, and sufficient properties in practical usewere obtained. Further, the transmittance wavefront aberration of thebroadband retarder 50, as a result of measuring with use of He—Ne laserlight having a wavelength of 633 nm, was 0.025 mλ_(rms) or less whichwas of sufficiently usable level as the optical element.

The broadband retarder 50 was assembled as a quarter-wave plate 51A inthe optical head device shown in FIG. 15. As a light source, asemiconductor laser source 511B having a wavelength band of 789 nm wasused, and as the other light source, a semiconductor laser source havinga wavelength band of 860 nm was disposed instead of the semiconductorlaser source 511A having a wavelength band of 660 nm. A non-linearoptical crystal KNbO₃ (not shown) is disposed between the lasersemiconductor laser source and a collimater lens 513A to convert thewavelength into 430 nm. As a result, satisfactorily circularly polarizedlight could be obtained with respect to the two kinds of linearlypolarized light of a wavelength band of 430 nm and a wavelength band of789 nm, and signal light having high efficiency of utilization of lightcould be obtained.

EXAMPLE 6

This example will be explained with reference to FIG. 23(a) and FIG.23(b). As shown in FIG. 23(a), a fixing substrate 504 made of glass withan antireflection film on its one surface to which laser light isincident (a lower surface in the figure) was prepared. A film ofpolyimide was formed on the surface at an optical disk side (an uppersurface in the figure) of the fixing plate 504, and the film wassubjected to a horizontally aligning treatment by rubbing, whereby apolyimide aligning film (not shown) was formed. On the glass substratesubjected to an aligning treatment, a thin film of organic material ofhorizontally aligned polymerized liquid crystal was formed in athickness of 3 μm. Then, a photolithography method and a dry etchingmethod were conducted to prepare an organic grating 507 of polymerizedliquid crystal having a pitch of 6 μm and a thickness of 3 μm.

As shown in FIG. 23(a), a broadband phase plate 51A was prepared bybonding a thin film of organic material 501B having birefringentproperties (linear expansion coefficient E₁=6.2×10⁻⁶/° C.) which had aretardation value of 180 nm obtained by stretching polycarbonate and athickness of 30 μm to a thin film of organic material 501A havingbirefringent properties which had a retardation value of 360 nm obtainedby stretching polycarbonate and a thickness of 30 μm by an adhesive 506wherein the thin film of organic material 501B was bonded on a fixingsubstrate 505 as a cover glass (linear expansion coefficientE₃=95×10⁻⁷/° C.) with the antireflection film on its upper surface by apolyester type UV curing adhesive 502 (linear expansion coefficientE₂=1.2×10⁻⁴/° C.). In this case, adjustment was made so that the fastaxis direction 509A of the thin film of organic material 501A was 60°with respect to the fast axis direction 509B of the thin film of organicmaterial 501B.

Here, definition was made so that the fast axis direction 509 of thebroadband phase plate was an intermediate direction between therespective fast axis directions of the two laminated thin films oforganic material. In measuring angles, a counterclockwise directionobserved from a side of the thin film of organic material 501 was takenas positive. Then, the surface at a side of the thin film of organicmaterial 501A of the broadband phase plate 51A and the surface at a sideof the organic grating 507 of the fixing substrate 504 of glass werebonded by an adhesive 503. In this case, arrangement was made so thatthe fast axis direction 509 of the broadband phase plate and thedirection of linearly polarized light of incident laser light in a goingpath formed an angle of 45°, and the adhesive 503 was filled in spacesof the organic grating of polymerized liquid crystal 507. The adhesive503 used here was an adhesive having a refractive index (n=1.5) whichwas equal to the ordinary refractive index n_(o) of polymerized liquidcrystal (ordinary refractive index n_(o)=1.5, extraordinary refractiveindex n_(e)=1.6) used for the organic grating. Finally, the laminate wascut by a dicing saw to prepare an optical element 51 in which thebroadband phase plate 51A having outer dimensions of 4 mm×4 mm and athickness of about 1.1 mm and the polarizing diffraction element 51Bwere unified.

Table 2 shows the optical characteristics of the thus prepared opticalelement in a wavelength band of 658 nm and a wavelength band of 787 nm.It was confirmed that an ellipse ratio angle of 85° or more could beobtained to any of the wavelength bands of 658 nm and 787 nm, and theoptical element functioned as a quarter-wave plate having a sufficientdurability in practical use.

TABLE 2 Ellipse ratio Wavelength angle of used for Transmittancetransmitted Diffraction measurement in going path light efficiency 658(nm) 98.0 (%) 87.5 (degree) 76 (%) 787 97.5 88.2 60

This optical element was designed so that the diffractioncharacteristics were optimum at a wavelength band of 660 nm.Accordingly, a sufficiently high transmittance could be obtained inpractical use although the diffraction characteristics in a wavelengthband of 790 nm were more or less lower than those in the wavelength bandof 660 nm. Further, the wavefront aberration of transmitted light wasgood as 0.025 λ_(rms) or less in a central portion (in a circle having adiameter of 2.5 mm) in the light incident/emitting plane of thepolarizing diffraction element.

This optical element 51 was installed in an actuator 517 for driving anobjective lens 515 in an optical head device as shown in FIG. 15. As aresult, sufficiently circularly polarized light was obtained withrespect to the linearly polarized light having two kinds of wavelength:a wavelength band of 658 nm and a wavelength band of 787 nm. Further,the polarizing diffraction element functioned sufficiently to suppressthe wavefront aberration, and signal light of extremely high efficiencyof utilization could be obtained.

EXAMPLE 7

Example 7 is a concrete example concerning a retarder 60 as shown inFIG. 16. On a transparent fixing substrate 604 of glass (linearexpansion coefficient E₃=95×10⁻⁷/° C.) having a refractive index of 1.5,a thin film of organic material 601B (linear expansion coefficientE₁=6.2×10⁻⁶/° C.) formed by stretching polycarbonate to providebirefringent properties was fixed by a polyester type UV curing adhesive602 (linear expansion coefficient E₂=1.2×10⁻⁴/° C.). The retardationvalue of R_(d) of the thin film of organic material 601B was 362 nm. Thevalue of R_(d) is half of the wavelength band value between a wavelengthband of 660 nm for DVD and a wavelength band of 790 nm for CD used forthe retarder. Then, a thin film of organic material 601A having the sameretardation value R_(d) of 362 nm as the above by stretchingpolycarbonate similarly to provide birefringent properties was fixed bya polyester type UV curing adhesive 606. In this case, the fixing wasconducted so that the optical axis of the thin film of organic material601A and the optical axis of the thin film of organic material 601Bformed an angle of 67.5°.

Further, a transparent fixing plate 605 having a refractive index of 1.5was bonded by using a polyester type UV curing adhesive 603 to therebyprepare the retarder 60. Linearly polarized light having two kinds ofwavelength band introduced from a side of the fixing substrate 604 ofthe thus prepared retarder 60 was emitted with their polarizationdirections rotated 45° in a counterclockwise direction in theobservation from a side of the fixing plate 605. On the other hand,linearly polarized light having two kinds of wavelength band introducedfrom a side of the fixing plate 605 of the phase plate 60 was emittedwith their polarization directions rotated 45° in clockwise direction inthe observation from a side of the fixing plate 605.

EXAMPLE 8

The retarder 60 prepared in Example 7 was disposed between a beamsplitter 614B and a collimater lens 613 of an optical head device asshown in FIG. 17. Two kinds of linearly polarized light whose the planesof polarized light were parallel to each other, emitted from asemiconductor laser source 611A having a wavelength band of 660 nm and asemiconductor laser source 611B having a wavelength band of 790 nm wereintroduced into the retarder 60 from a side of a transparent fixingplate 604 (FIG. 16). The retarder 60 was disposed in the optical headdevice so that the polarization direction of laser light and the opticalaxis of the thin film of organic material 601B (FIG. 16) constitutingthe retarder 60 formed an angle of 55°. In the optical head device thusformed, light having a wavelength band of 660 nm and light having awavelength band of 790 nm converged on an optical disk 616 were bothlinearly polarized light having their polarization directions being inparallel to each other.

FIG. 24 shows the wavelength dependence of ellipse ratio angles as aparameter indicating the linearity of polarized light. The solid line(A) in FIG. 24 shows the wavelength dependence of ellipse ratio anglesof light transmitting through the retarder 60 and converged on theoptical disk in this example, and shows substantially linearly polarizedlight in a broad wavelength band of 600-900 nm. On the other hand, thedotted line (B) in FIG. 24 shows the wavelength dependence of ellipseratio angles of light transmitting through a conventional half-waveplate to be converged on the optical disk designed with respect to awavelength of 730 nm. The region of linearly polarized light (a regionof zero ellipse ratio angle) is very narrow.

FIG. 25 shows the wavelength dependence of polarization directions oflight converged on the optical disk. The polarization direction means along axis direction of elliptically polarized light. It is found fromFIG. 25 that the polarization direction of light (solid line (A))transmitting through the retarder 60 to be converged on the optical diskin this example and the polarization direction of light (dotted line(B)) transmitting through the conventional half-wave plate to beconverged on the optical disk which was designed with respect to lighthaving a wavelength band of 730 nm are not substantially different, andthat both the polarization direction of the light having a wavelengthband of 660 nm and the polarization direction of the light having awavelength band of 790 nm are substantially 45°.

EXAMPLE 9

Example 9 is a concrete example of the optical element 71 shown in FIG.19. On a transparent fixing substrate 704 made of glass (linearexpansion coefficient E₃=95×10⁻⁷/° C.) having a refractive index of 1.5,a thin film of organic material 701 made of polycarbonate (linearexpansion coefficient E₁=6.2×10⁻⁶/° C.) having a retardation value of1650 nm to light having a wavelength band of 660 nm was fixed by anacrylic type adhesive 702 (linear expansion coefficient E₂=1.1×10⁻⁴/°C.). The phase difference produced in this case was 5π in 2π(m₁−½) wherem₁=3. Further, the phase difference corresponding to the above-mentionedphase difference, with respect to linearly polarized light having awavelength band of 790 nm was 4π in 2πm₂ where m₂=2. Then, two kinds oflinearly polarized light having a wavelength band of 660 nm and awavelength band of 790 nm were incident into the thin film of organicmaterial with their polarization directions inclined 45° with respect tothe fast axis direction of the thin film of organic material.

With such phase differences, the planes of polarized light of the twokinds of light having different wavelength bands cross perpendicularlyafter they have transmitted through the thin film of organic material701 of polycarbonate because the polarization plane of the linearlypolarized light having a wavelength band of 660 nm was rotated 90° andthe polarization plane of the linearly polarized light having awavelength band of 790 nm was not rotated.

On the other hand, polyimide for an aligning film was coated on atransparent fixing substrate 705 having a refractive index of 1.5 toform a film. An aligning treatment was conducted to the film by rubbingand then, a film of polymerized liquid crystal was formed on the alignedfilm. The refractive index of the polymerized liquid crystal was about1.6 in terms of the extraordinary refractive index n_(e) and about 1.5in terms of the ordinary refractive index n_(o) after curing byphotopolymerization. For the purpose of obtaining desired diffractionangle and diffraction efficiency to light having a wavelength band of790 nm incident obliquely to the optical element 71, a transparentsubstrate having such function that only one kind of polarized light wasdiffracted between two perpendicularly crossing polarized light wasprepared by conducting alternately photolithography and etching threetimes to thereby process the polymerized liquid crystal into a saw-teethlike grating 706 having 8 steps. The transparent substrate does not havea diffracting effect to light having a wavelength band of 660 nm.

The optical element 71 having such diffracting function that only lighthaving a wavelength band of 790 nm was diffracted to make the opticalaxis of the light coincident with the optical axis of light having awavelength band of 660 nm was prepared by fixing the thin film oforganic material 701 to the transparent substrate so as to oppose thesaw-teeth like grating 706 by using an acrylic type transparent fillingadhesive 703 having a refractive index n_(o) which was substantiallyequal to the ordinary refractive index n_(o) of the polymerized liquidcrystal.

EXAMPLE 10

The optical element 71 having a deflecting function to light having awavelength band of 790 nm, prepared in Example 9, was disposed between atwo-wavelength semiconductor laser source 711 and a beam splitter 714 inan optical head as shown in FIG. 20. The positions of light emittingpoints 711A and 711B generating two-wavelengths were separated from eachother so that the respective optical axes were not coincident with eachother. However, with use of the optical element 71, only light having awavelength band of 790 nm was deflected so that it was made coincidentwith the optical axis of light having a wavelength band of 660 nm. As aresult, a small wavefront aberration and excellent on-axial performancecould be obtained, and good reproducing and recording could be carriedout to optical disks of DVD and CD by using a photodetector 712 having asmall light receiving surface area, commonly used in recording andreproducing information in the optical disks of CD and DVD.

Further, instead of the saw-teeth like grating 706 (FIG. 19) having asimple linear grating pattern, a hologram pattern wherein the pitch ofgrating and the direction of grating are changed in a surface area maybe formed whereby a spatial distribution of phase is given to diffractedlight having a wavelength band of 790 nm; a phase difference of thediffracted light can be adjusted, and light converging properties to anoptical disk can further be improved. Specifically, the coma aberrationresulted in diffracted light having a wavelength band of 790 nm becauseof a difference between the optical axis of the light having awavelength band of 790 nm and the optical axis of incident light havinga wavelength band of 660 nm, or the chromatic aberration or thespherical aberration resulted from a wavelength dispersion of therefractive index of the optical element used in the optical head device,due to a difference between a wavelength band of 660 nm and wavelengthband of 790 nm, can be corrected.

Further, the same effect as above can be obtained by disposing theoptical element 71 between the beam splitter 714 and the photodetector712 in this optical head.

EXAMPLE 11

Example 11 is a concrete example of the optical element shown in FIG. 9,and an application example having a structure that the phase platecomprising the thin film of organic material described in Example 6 anda diffraction grating are unified. FIG. 26 shows the structure of anoptical element 80 in cross section according to this example. A thinfilm of organic material 801 made of polycarbonate (linear expansioncoefficient E₁=6.2×10⁻⁶/° C.) having a retardation value of 181 nm wasinterposed between transparent fixing substrates 804, 805 made of quartzglass (linear expansion coefficient E₃=5×10⁻⁷/° C.) having a diffractiveindex of 1.46 so that the phase difference produced in light having awavelength of 725 nm as an intermediate wavelength between a wavelengthband of λ₁=660 nm and a wavelength band of λ₂=790 nm was π/2, and theywere fixed by acrylic type adhesives layers 802, 803 (linear expansioncoefficient E₂=1.1×10⁻⁴/° C.), whereby a phase plate as the opticalelement 80 was prepared.

In this example, a diffraction grating 808A and a diffraction grating808B each comprising a concave and convex portion having a uniformdiffractive index are formed on surfaces of the fixing substrates 804,805. The depth of grating d₁ of the concave and convex portion of thediffraction grating 808B is d₁≈λ₁/(n₁−1) where the refractive index of aconvex portion is n₁, and the depth of grating d₂ of the concave andconvex portion of the diffraction grating 808A is d₂≈λ₂/(n₂−1) where therefractive index of a convex portion is n₂. Specifically, the surfacesof the fixing substrates 805, 804 were etched directly to obtain d₁=1.43μm and d₂=1.72 μm. As a result, in the diffraction grating 808B, thephase difference in the concave and convex portion is 2π to thewavelength λ₁ but are not 2π to the wavelength λ₂. Further, in thediffraction grating 808A, the phase difference in the concave and convexportion is 2π to the wavelength λ₂ but not 2π to the wavelength λ₁.

Accordingly, a wavelength-selective diffraction grating having differentdiffracting functions to light having different wavelengths can berealized. Namely, as shown in FIG. 26, the diffraction grating 808Aperforms the diffracting function to incident light of wavelength λ₁,and produces the 0th-order light of about 71% and the ± first-orderlight of about 10%. On the other hand, the diffracting grating 808Bperforms the diffracting function to incident light of wavelength λ₂,and produces the 0th-order light of about 63% and the ± first-orderlight of about 13%.

This optical element 80 was disposed between two-wavelengthsemiconductor laser source 711 and the beam splitter 714 at an emittingside (not shown) of the optical element 71 in the optical head deviceshown in FIG. 20. Light of wavelength λ₂ emitted from the two-wavelengthsemiconductor laser source 711 is separated into 3 beams in total: onekind of the 0th-order light and two kinds of the ± first-order light bythe optical element 80, which are used for detecting recordedinformation and tracking signals in the optical disk 716 of CD by a 3beam method. Further, light of wavelength λ₁ emitted from thetwo-wavelength semiconductor laser source 711 is separated into 3 beamsin total: the 0th-order light and the ± first-order light by the opticalelement 80 which are used for detecting recorded information andtracking signals in the optical disk 716 of DVD by a differentialpush-pull method.

Thus, in the optical head device installing the optical element 80 ofthis example, light of wavelength λ₁ and light of wavelength λ₂ arediffracted independently by the diffraction gratings 808A, 808B andaccordingly, there is no reduction of efficiency and no stray light.Therefore, the recording and reproducing of information in optical disksof DVD and CD can stably be conducted.

Further, linearly polarized light having the wavelength λ₁ and thewavelength λ₂ transmitting through the optical element 80 are renderedto be substantially circularly polarized light by a phase plate 801(FIG. 26) comprising a thin film of organic material having birefringentproperties. Here, the phase plate 801 is fixed to the fixing substrates804, 805 by the adhesive layers 802, 803. Accordingly, returning lightreflected by the information recording surface of the optical disk andtransmitting through the beam splitter 714 transmits again through theoptical element 80, whereby the direction of linearly polarized light ofthe returning light is rendered to be perpendicular to the direction oflinearly polarized light emitted from the laser oscillating source.Then, the returning light is incident into light emitting points of thesemiconductor laser source. Accordingly, there is no interferencebetween the returning light from the optical disk and the oscillatedlaser light, and no fluctuation of an oscillated output takes place,whereby the recording and reproducing of information in the optical diskcan stably be conducted.

In this example, description has been made as to the optical element 80in which the diffraction grating 808A and the diffraction grating 808Bare formed. However, an element structure provided with eitherdiffraction grating may be used. Further, the broadband retarder 50according to the seventh embodiment shown in FIG. 10 may be used insteadof the above-mentioned retarder 80A. In this case, the broadbandretarder 50 functions as a quarter-wave plate to the wavelength λ₁ andthe wavelength λ₂. Accordingly, the interference between the returninglight from the optical disk and the oscillated laser light can furtherbe reduced, and therefore, stability in recording and reproducinginformation in the optical disk can further be increased.

EXAMPLE 12

Example 12 is an application example of the optical element shown inFIG. 9 or FIG. 18, which has such structure described in Example 6 andExample 9 that the phase plate comprising the thin film of organicmaterial and a polarizing diffraction grating are unified. FIG. 27 showsthe structure of an optical element 81 in cross section of this example.In FIG. 27, the structure of a retarder 80A is the same as that of theoptical element 70 in Example 9. A thin film of organic material 801made of polycarbonate (linear expansion coefficient E₁=6.2×10⁻⁶/° C.)was interposed between fixing substrates 804, 805 made of glass (linearexpansion coefficient E₃=95×10⁻⁷/° C.) and they were fixed by acrylictype adhesive layers 802, 803 (linear expansion coefficientE₂=1.1×10⁻⁴/° C.). The thin film of organic material 801 was adapted sothat the polarization plane of linearly polarized light having awavelength band of λ₁=660 nm was rotated 90° and the polarization plateof linearly polarized light having a wavelength band of λ₂=790 nm wasnot rotated. Accordingly, after the light having two-wavelength bandswhose the planes of polarized light were parallel to each other hadtransmitted through the phase plate, their planes of polarized lightwere crossed perpendicularly. In this case, the two kinds of linearlypolarized light having the wavelength λ₁ and the wavelength λ₂ wereincident into the thin film of organic material with their polarizationdirections inclined 45° with respect to the fast axis direction of thethin film of organic material.

On the other hand, polarizing diffraction gratings 807A, 807B comprisingpolymerized liquid crystal formed on the fixing substrates 805, 809 ofglass were prepared by the same manufacturing process as in Example 9.Here, the diffraction grating 807A was a linear grating of rectangularshape in cross section wherein the direction of polymerized liquidcrystal was aligned so that diffracted light was produced with respectto linearly polarized light of wavelength λ₂ and diffracted light wasnot produced with respect to linearly polarized light of wavelength λ₁which crossed perpendicular to the linearly polarized light ofwavelength λ₂. On the other hand, the diffraction grating 807B was alinear grating of rectangular shape in cross section wherein the aligneddirection of polymerized liquid crystal was perpendicular to the aligneddirection of the polymerized liquid crystal constituting the diffractiongrating 807A so that diffracted light was produced with respect tolinearly polarized light of wavelength λ₁ and diffracted light was notproduced with respect to linearly polarized light of wavelength λ₂. Thefixing substrates 805, 809 on which the diffraction gratings 807A, 807Bwere formed were bonded by using a filling adhesive 806 having arefractive index which was substantially equal to the ordinaryrefractive index of the polymerized liquid crystal.

Linearly polarized light having a wavelength λ₁ and a wavelength λ₂whose the planes of polarized light were parallel to each other wereintroduced into the optical element 81 having such structure. Then, thediffraction gratings 807A and 807B could produce respectively the0th-order light and the ± first-order diffracted light having awavelength λ₁ and a wavelength λ₂ independently. The pitch of gratingand the steps of grating d₁ and d₂ of the diffraction grating 807A andthe diffraction grating 807B were so determined that a desireddiffraction angle and diffraction efficiency could be obtained.

The optical element 81 of this example was disposed instead of theoptical element 71 between the two-wavelength semiconductor laser source711 and the beam splitter 714 of the optical head device shown in FIG.20. Light having a wavelength 12 emitted from the two-wavelengthsemiconductor laser source 711 is separated into 3 beams in total: onekind of the 0th-order light and two kinds of the ± first-order light bythe optical element 81, which are used for detecting recordedinformation and tracking signals in the optical disk 716 of CD by 3 beammethod. Further, light having a wavelength λ₁ emitted from thetwo-wavelength semiconductor laser source 711 is separated into 3 beamsin total: the 0th-order light and the ± first-order light by the opticalelement 81, which are used for recorded information and tracking signalsin the optical disk 716 of DVD by a differential push-pull method.

Unlike Example 11, since there is no restriction for the steps ofgrating d₁ and d₂ of the diffraction gratings 807B and 807A, whichproduce a phase difference of 2π with respect to either wavelength,designing can be flexible to obtain desired diffraction efficiency withrespect to wavelengths functioning as diffraction grating. Such featureimproves optically recording efficiency to an optical disk in an opticalhead device for recording information, and accordingly, it is suitablefor a case that the efficiency of the 0th-order light be 80% or more andthe ± first-order diffraction light be reduced.

In FIG. 20, the case that the optical element 81 is disposed in place ofthe optical element 71 has been explained. However, the optical element81 may be disposed at a light emission side of the optical element 71.In this case, since incident light having a wavelength λ₁ and awavelength λ₂ to the optical element 81 have already beenperpendicularly crossing polarized light, the phase plate 801 is notused at a light incident side in the structure of the optical element 81in FIG. 27. Instead, when a phase plate comprising a thin film oforganic material (not shown) made of polycarbonate having a retardationvalue of 181 nm is unified with the optical element at its lightemission side in the same manner as in Example 11, the interferencebetween returning light from the optical disk and oscillated laser lightcan be reduced, and the recording and reproducing of information in theoptical disk can stably be conducted.

INDUSTRIAL APPLICABILITY

As described above, according to the present invention, an adhesive iscoated on at least one surface of a thin film of organic material havingbirefringent properties and a phase-difference producing function; afixing substrate having a transmitting or reflecting function is bondedto the thin film of organic material by the adhesive, and materialswherein relations of E₁<E₂ and E₃<E₂ where E₁ is the linear expansioncoefficient of the thin film of organic material, E₂ is the linearexpansion coefficient of the adhesive and E₃ is the linear expansioncoefficient of the fixing substrate, and the glass transitiontemperature of the thin film of organic material is 150° C. or more, areselected. Accordingly, the temperature characteristics of a phasedifference can be obtained in the direction of suppressing a temperaturechange of oscillated wavelength from a semiconductor laser source, andtherefore, a phase difference can be kept to a constant value even whenthere is a fluctuation of the wavelength of emitted light from asemiconductor laser source due to a temperature change. In particular, apredetermined phase-difference producing function can be maintainedstably even in a high temperature region. Further, expansions of thethin film of organic material and the fixing substrate with atemperature rise can be absorbed by the adhesive, and a deformation ofthe retarder can be prevented. Accordingly, a retarder free from achange of phase difference, a change of the retardation axis directionand a disturbance of the wavefront of laser light transmitting throughthe retarder can be provided.

Further, by combining a polarizing diffraction element having differentdiffraction efficiencies depending on polarization directions ofincident light with the above-mentioned phase plate, a fluctuation ofthe oscillated wavelength of semiconductor laser due to temperature canbe compensated in the retarder, and accordingly, a change of diffractionefficiency due to temperature in the diffraction element can beprevented. As a result, an optical element minimizing the transmissionwavefront aberration and being compact, and having both functions as apolarizing diffraction element and retarder can be obtained.

Further, by giving at least one function among an aperture controllingfunction, a wavefront aberration correcting function and a diffractingfunction to at least one of transparent substrates as fixing substratesof the retarder, an optical element having a plurality of functions suchas a phase-difference producing function can be formed. Further, byassembling the optical element in an optical head device, the number ofparts constituting the optical head device can be reduced, and thesize-reduction can be achieved.

Further, a broadband phase plate is disposed between a light source andan objective lens in an optical head device, and the broadband phaseplate comprises two phase plates which are piled up so that respectiveoptical axes cross to each other wherein the retardation value of thephase plate to which laser light is first incident, is larger than theretardation value of the phase plate to which laser light is incidentsecondly, and the ratio of these retardation values is 1.8-2.2. Withthis structure, the broadband phase plate functions as a quarter-waveplate with respect to transmitting laser light which is polarizedlinearly and has different wavelengths, whereby linearly polarized lightcan be transformed into circularly polarized light. Accordingly, byassembling the broadband phase plate in the optical head device, signallight detected as reflected returning light having different wavelengthsfrom the optical disk becomes signal light having high efficiency ofutilization of light.

Further, the retarder of the present invention can provide the samephase difference with respect to laser light having both wavelengths forCD and DVD while a state of linear polarization is maintained, and thepolarization directions of the laser light having the both wavelengthscan be rotated by the same angle. By installing such retarder in theoptical head device, the optical head device which minimizes error inrecording or reproducing information in the optical disk and detectssignal stably, can be realized.

Further, in the retarder of the present invention, linearly polarizedlight having two different kinds of wavelength whose the planes ofpolarized light are in parallel to each other can be crossedperpendicularly. By adding a polarizing diffracting function to suchretarder, a diffraction element having wavelength selectivity can beobtained. Further, by installing it in an optical head device having atwo-wavelength semiconductor laser source in which positions of lightemitting points are slightly different, the optical axes of two kinds oflinearly polarized light can be agreed with each other. Further, 3 beamsfor detecting signals can be formed for each wavelength. As a result,each optical part in the device can perform good optical performance,and optical head device which can detect signals stably and lessen errorin recording or reproducing information by using a photodetector used incommon for two-wavelengths, can be realized, and size-reduction and theweight-reduction can be realized.

1. An optical head device comprising: a semiconductor laser light sourcefor emitting laser lights having two or more different wavelengths, anobjective lens for converging the laser lights emitted from thesemiconductor laser light source, an optical recording medium to whichthe laser lights are converged and introduced, a photodetector forreceiving reflected light from the optical recording medium, and abroadband retarder located in an optical path from the laser lightsource to the optical recording medium, or an optical path from theoptical recording medium to the photodetector to control a state ofphase of the laser lights, the broadband retarder having such structurethat a phase plate A to which any one of the laser lights is firstincident and a phase plate B to which any one laser light is secondlyincident are piled up so that respective fast axes of optical axes ofthe phase plates A and B are crossed wherein the ratio of theretardation value of the phase plate A to the retardation value of thephase plate B is 1.8-2.2.
 2. The optical head device according to claim1, wherein the laser light is two kinds of laser having differentwavelengths, the ratio of the retardation value is 2, and the degrees ofelliptical polarization when the two kinds of laser light aretransmitted through the broadband retarder are substantially equal.
 3. Aretarder comprising: a thin film of organic material having birefringentproperties, an adhesive coated on at least one surface of the thin filmof organic material, and a fixing substrate bonded to the thin film oforganic material by the adhesive, wherein among the linear expansioncoefficient E₁ of the thin film of organic material, the linearexpansion coefficient E₂ of the adhesive and the linear expansioncoefficient E₃ of the fixing substrate, relations of E₁<E₂ and E₃<E₂ aresatisfied.
 4. The retarder according to claim 3, wherein the thin filmof organic material includes at least one selected from the groupconsisting of polycarbonate, polyimide, polyallylate, polyethersulfone,(alicyclic) polyolefin, poly(meth)acrylate, polyetheriinide andpolymerized liquid crystal.
 5. The retarder according to claim 3,wherein the adhesive includes at least one selected from the groupconsisting of acryl type, epoxy type, urethane type and polyester type.6. An optical head device comprising: a semiconductor laser lightsource, an objective lens for converging laser light emitted from thesemiconductor laser light source, an optical recording medium to whichthe laser light is converged and introduced, a photodetector forreceiving reflected light from the optical recording medium and, anoptical element fabricated by combining a retarder as described in claim3 with a polarizing diffraction element having different diffractionefficiencies depending on a state of polarization of incident lightwherein the optical element is located in an optical path from the laserlight source to the optical recording medium, or an optical path fromthe recording medium to the photodetector.
 7. An optical element whereina structure having at least one element among the following threeelements is fanned on a fixing substrate on which a retarder asdescribed in claim 3 is formed: (1) an aperture controlling elementprovided with a first region in a central portion, which transmits lighthaving two or more kinds of wavelength and a second region surroundingthe first region, which reflects or diffracts light having one or morekinds of wavelength, (2) a retarder having a ringed belt-like groove forcorrecting the wavefront of transmitted light in a central portion,which transmits light having two or more kinds of wavelength, and (3) adiffraction element having a periodic concave and convex portion incross-sectional view, which diffracts incident light.
 8. In an opticalhead device wherein laser light emitted from a semiconductor lasersource is converged by an objective lens to be introduced into anoptical recording medium, and reflected light from the optical recordingmedium is received by a photodetector, the optical head device beingcharacterized in that the optical element described in claim 7 islocated in an optical path from the laser light source to the opticalrecording medium, or a light path from the optical recording medium tothe photodetector.
 9. An optical head device comprising: a semiconductorlaser light source for emitting linearly polarized light havingwavelengths of λ₁ and λ₂ (λ₁<λ₂), an objective lens for converging thelaser light emitted from the semiconductor laser source, an opticalrecording medium to which the laser light is converged and introduced, aphotodetector for receiving reflected light from the optical recordingmedium, and an retarder described in claim 3 located in an optical pathfrom the laser light source to the optical recording medium wherein thelinearly polarized light having wavelengths of λ₁ and λ₂ are incident tothe retarder, two thin films of organic material having birefringentproperties each having a retardation value of λ/2 with respect tolinearly polarized light having a wavelength of λ in a relation ofλ₁≦λ≦λ₂ are piled up so that the respective optical axes are crossed,and when the linearly polarized light having wavelengths of λ₁ and λ₂are transmitted through the thin films of organic material, the planesof polarized light provided by the linearly polarized light are rotatedby the same angle.
 10. An optical head device comprising: asemiconductor laser light source for emitting two kinds of linearlypolarized light having different wavelengths and the planes of polarizedlight in parallel to each other, an objective lens for converging thelaser light emitted from the semiconductor laser source, an opticalrecording medium to which the laser light is converged and introduced, aphotodetector for receiving reflected light from the optical recordingmedium, and an retarder described in claim 3 located in an optical pathfrom the laser light source to the optical recording medium, or anoptical path from the optical recording medium to the photodetector,wherein the retarder comprises a thin film of organic material toproduce a phase difference of 2π(m₁−½) (m₁ is a natural number) withrespect to a kind of linearly polarized light and a phase difference of2π m₂ (m₂ is a natural number) with respect to the other kind oflinearly polarized light when the two kinds of linearly polarized lighthaving different wavelenthgs are transmitted therethrough with aninclination of 45° in a polarization direction in its fast axisdirection, whereby the planes of polarized light provided by thelinearly polarized light of two kinds of wavelengths crossperpendicularly.